[azure-ai-textanalytics] recognize_pii_entities: per-category recognition criteria (e.g., BRCPFNumber checksum) are undocumented

> **Note on test data used below:** Neither CPF value in this report is a real person's identifier.
> - `123.456.789-00` is syntactically a CPF but is mathematically invalid (it fails the public Brazilian check-digit algorithm — all digits sequential, check digits zero). It is a non-issuable placeholder commonly used as obvious dummy data.
> - `998.214.865-68` is a valid-formatted CPF but is not a real person's number — it is the exact value Microsoft itself ships in this very SDK's official sample [`sample_recognize_pii_entities.py`](https://github.com/Azure/azure-sdk-for-python/blob/main/sdk/textanalytics/azure-ai-textanalytics/samples/sample_recognize_pii_entities.py) (`"...Brazilian CPF number 998.214.865-68"`). I reuse it here only to keep the repro identical to the official sample.

- **Package Name:** azure-ai-textanalytics
- **Package Version:** latest
- **Operating System:** Windows
- **Python Version:** 3.x

## Describe the issue

The docstring and reference docs for [`TextAnalyticsClient.recognize_pii_entities`](https://learn.microsoft.com/en-us/python/api/azure-ai-textanalytics/azure.ai.textanalytics.textanalyticsclient#azure-ai-textanalytics-textanalyticsclient-recognize-pii-entities) and the `categories_filter` keyword describe the **request shape** (which categories to filter for) but give no signal that **detection criteria differ per category** — some appear to do format-only matching, others apparently apply additional validation (e.g., checksum) that is not documented.

This is more of a clarification request than a bug.

## To Reproduce

```python
from azure.ai.textanalytics import TextAnalyticsClient
from azure.core.credentials import AzureKeyCredential

client = TextAnalyticsClient(endpoint, AzureKeyCredential(key))

# (A) syntactically valid CPF format, invalid check digits -> NOT detected
res_a = client.recognize_pii_entities(
["Entre em contato pelo CPF 123.456.789-00"], language="pt"
)
# (B) valid CPF (same value used in this SDK's official PII sample) -> detected as BRCPFNumber
res_b = client.recognize_pii_entities(
["Entre em contato pelo CPF 998.214.865-68"], language="pt"
)
```

No `categories_filter` is passed in either case, so the default detection set applies. (A) returns no `BRCPFNumber`; (B) does.

## Expected behavior

Either:

1. The SDK reference / conceptual doc points out that recognition criteria vary per category and may include semantic validation beyond format (and points to a service-level page that lists per-category criteria), **or**
2. The service-level doc page ([Recognized PII and PHI entities](https://learn.microsoft.com/en-us/azure/ai-services/language-service/personally-identifiable-information/concepts/entity-categories)) is updated and the SDK references it.

> Note: I attempted to file the documentation-side report directly against `MicrosoftDocs/azure-docs`, but that repository now has GitHub Issues disabled (`has_issues: false`), so this SDK issue is the only public channel available for the report. Routing the service-side fix internally would be appreciated.

## Why this matters for SDK users

Without this signal, developers writing tests with placeholder PII (a common pattern) get silent false negatives and have to reverse-engineer the detection criteria empirically. This was the path I took to discover the behavior.

Start with TextAnalyticsClient.recognize_pii_entities and its categories_filter reference, then compare the official sample at sdk/textanalytics/azure-ai-textanalytics/samples/sample_recognize_pii_entities.py with the Recognized PII and PHI entities service page. Done means the SDK or service documentation explains that recognition criteria vary by category and may include validation beyond format, with the relevant cross-reference.

Scritto dal modello di indicizzazione a partire dal testo della issue.