[docs] fix documentation inconsistencies in docstring and mgmt tests guides

## Summary

This PR fixes three documentation inconsistencies found during a review of all markdown files under `doc/`.

### Changes

#### `doc/dev/mgmt/tests.md` — wrong `ClientSecretCredential` parameter names

The first code snippet demonstrating `ClientSecretCredential` used `secret=` and `tenant=` as keyword argument names. These parameters do not exist in the actual constructor:

```python
# ❌ Before (wrong parameter names)
credentials = ClientSecretCredential(
client_id = os.environ['AZURE_CLIENT_ID'],
secret = os.environ['AZURE_CLIENT_SECRET'],
tenant = os.environ['AZURE_TENANT_ID']
)
```

```python
# ✅ After (correct parameter names, ordered to match the positional signature)
credentials = ClientSecretCredential(
tenant_id = os.environ['AZURE_TENANT_ID'],
client_id = os.environ['AZURE_CLIENT_ID'],
client_secret = os.environ['AZURE_CLIENT_SECRET']
)
```

The actual constructor signature (from `sdk/identity/azure-identity/azure/identity/_credentials/client_secret.py`) is:
```python
def __init__(self, tenant_id: str, client_id: str, client_secret: str, **kwargs: Any) -> None:
```

The second snippet in the same file already used the correct names (`client_secret=`, `tenant_id=`) and was left unchanged.

#### `doc/dev/mgmt/tests.md` — reference to non-existent `AzureTestCase` class

Line 118 referenced `AzureTestCase`, which does not exist in `devtools_testutils`. The correct class name is `AzureMgmtRecordedTestCase` (defined in `eng/tools/azure-sdk-tools/devtools_testutils/mgmt_recorded_testcase.py`).

#### `doc/dev/docstring.md` — example uses deprecated comment-style type annotation

The example method signature used a Python 2-era comment-style type annotation:
```python
def begin_training(self, training_files_url, use_training_labels, **kwargs):
# type: (str, bool, Any) -> LROPoller[CustomFormModel]
```

This contradicts the guidance in `doc/dev/static_type_checking.md`, which explicitly states:
> "Do not use comment style type hints (`# type: str`). Some of our libraries use type comments due to legacy code supporting Python 2, but these will be updated to annotation style."

The example has been updated to use PEP 484 inline annotation style:
```python
def begin_training(self, training_files_url: str, use_training_labels: bool, **kwargs: Any) -> "LROPoller[CustomFormModel]":
```

> Generated by [Documentation Consistency Check](https://github.com/Azure/azure-sdk-for-python/actions/runs/28771515398) · 288.4 AIC · ⌖ 13.4 AIC · ⊞ 5.8K · [◷](https://github.com/search?q=repo%3AAzure%2Fazure-sdk-for-python+%22gh-aw-workflow-id%3A+doc-consistency-check%22&type=pullrequests)

---

> [!NOTE]
> This was originally intended as a pull request, but GitHub Actions is not permitted to create or approve pull requests in this repository.
> The changes have been pushed to branch `docs/fix-inconsistencies-2026-07-260da81426ba64e7`.
>
> **[Click here to create the pull request](https://github.com/Azure/azure-sdk-for-python/compare/main...docs/fix-inconsistencies-2026-07-260da81426ba64e7?expand=1&title=%5Bdocs%5D%20fix%20documentation%20inconsistencies%20in%20docstring%20and%20mgmt%20tests%20guides)**

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Show patch preview (71 of 71 lines)

```diff
From abf547d441064f8bc3b497fd6c93cac436ea552a Mon Sep 17 00:00:00 2001
From: "github-actions[bot]"
Date: Mon, 6 Jul 2026 06:21:13 +0000
Subject: [PATCH] docs: fix documentation inconsistencies in docstring and mgmt
tests guides

- doc/dev/mgmt/tests.md: Fix ClientSecretCredential parameter names in the
first example code snippet. The parameters 'secret' and 'tenant' do not
exist in the actual constructor; correct names are 'client_secret' (str)
and 'tenant_id' (str). Also reorder args to match the positional signature:
(tenant_id, client_id, client_secret).

- doc/dev/mgmt/tests.md: Replace reference to non-existent 'AzureTestCase'
class with the correct 'AzureMgmtRecordedTestCase' class from
devtools_testutils.mgmt_recorded_testcase.

- doc/dev/docstring.md: Update the example method signature to use PEP 484
annotation-style type hints instead of the deprecated comment-style
'# type: (str, bool, Any) -> ...' annotation. This aligns the example
with the guidance in static_type_checking.md which explicitly states
'Do not use comment style type hints'.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
---
doc/dev/docstring.md | 3 +--
doc/dev/mgmt/tests.md | 6 +++---
2 files changed, 4 insertions(+), 5 deletions(-)

diff --git a/doc/dev/docstring.md b/doc/dev/docstring.md
index 876c67c7..4b69af1f 100644
--- a/doc/dev/docstring.md
+++ b/doc/dev/docstring.md
@@ -16,8 +16,7 @@ Docstrings are noted by the Python long-string `""""""`. When adding
A method docstring is annotated by the Python long-string `""""""` right after the method definition. The convention is a short line ending with a period, two new lines, followed by a longer description. Below is an example of a full docstring from `azure-ai-formrecognizer`:
```python
@distributed_trace
- def begin_training(self, training_files_url, use_training_labels, **kwargs):
- # type: (str, bool,
... (truncated)
```

Review the three examples in doc/dev/mgmt/tests.md against the ClientSecretCredential signature in sdk/identity/azure-identity/azure/identity/_credentials/client_secret.py and the AzureMgmtRecordedTestCase definition in eng/tools/azure-sdk-tools/devtools_testutils/mgmt_recorded_testcase.py. Then check doc/dev/docstring.md against doc/dev/static_type_checking.md. Done means the parameter names, test-case class reference, and type annotation style are consistent.

Written by the indexing model from the issue text.