We don't support sni cert for aio. Remove the doc. (#35475)

* We don't support sni cert for aio. Remove the doc.

* update readme

Author: xiangyan99

sdk/identity/azure-identity/README.md

@@ -294,7 +294,7 @@ variables:
 |`AZURE_TENANT_ID`|ID of the application's Microsoft Entra tenant|X
 |`AZURE_CLIENT_CERTIFICATE_PATH`|path to a PEM or PKCS12 certificate file including private key|X
 |`AZURE_CLIENT_CERTIFICATE_PASSWORD`|password of the certificate file, if any|
-|`AZURE_CLIENT_SEND_CERTIFICATE_CHAIN`|if True, the credential will send the public certificate chain in the x5c header of each token request's JWT. This is required for Subject Name/Issuer (SNI) authentication. Defaults to False.|
+|`AZURE_CLIENT_SEND_CERTIFICATE_CHAIN`|if True, the credential will send the public certificate chain in the x5c header of each token request's JWT. This is required for Subject Name/Issuer (SNI) authentication. Defaults to False. There is a [known limitation](https://github.com/Azure/azure-sdk-for-python/issues/13349) that async SNI authentication is not supported.|
 
 ### Username and password
 

sdk/identity/azure-identity/azure/identity/aio/_credentials/environment.py

@@ -36,9 +36,6 @@ class EnvironmentCredential(AsyncContextManager):
       - **AZURE_CLIENT_ID**: the service principal's client ID
       - **AZURE_CLIENT_CERTIFICATE_PATH**: path to a PEM or PKCS12 certificate file including the private key.
       - **AZURE_CLIENT_CERTIFICATE_PASSWORD**: (optional) password of the certificate file, if any.
-      - **AZURE_CLIENT_SEND_CERTIFICATE_CHAIN**: (optional) If True, the credential will send the public certificate
-        chain in the x5c header of each token request's JWT. This is required for Subject Name/Issuer (SNI)
-        authentication. Defaults to False.
       - **AZURE_AUTHORITY_HOST**: authority of a Microsoft Entra endpoint, for example
         "login.microsoftonline.com", the authority for Azure Public Cloud, which is the default
         when no value is given.