Improve Form Recognizer endpoint documentation

There are two action items for this issue:

**1. Fix client documentation:**
Check the `azure-ai-formrecognizer` client documentation to make sure the description for the `endpoint` parameter is clear. Current documentation is using the regional form recognizer endpoint as an example for users which is not correct, we should instead be specifying the **resource endpoint** in our documentation and examples.

Current documentation:
```
:param str endpoint: Supported Cognitive Services endpoints (protocol and hostname,
for example: https://westus2.api.cognitive.microsoft.com).
```

Expected documentation:
```
:param str endpoint: Supported service endpoints (protocol and hostname,
for example: https://.cognitiveservices.azure.com/).
```

A regional endpoint looks like: https://.api.cognitive.microsoft.com/
A resource endpoint looks like: https://.cognitiveservices.azure.com/

**2. Fix README:**
In the authentication section of the readme, we have a link that points to the regional endpoints, but the link isn't clearly listing regional endpoints anymore for Form Recognizer. We should consider removing this information from the readme and just have folks use the resource endpoints for authentication and client initialization. This is where the link is:


Link: https://azure.microsoft.com/en-us/explore/global-infrastructure/products-by-region/?products=form-recognizer