document that credential needs to be from azure.identity.aio (#463)

* document that `credential` needs to be from azure.identity.aio

if it's a sync credential, then the weakref.finalize call will
cause an error, close() -> None cannot be awaited.

* Update adlfs/spec.py

* Update adlfs/spec.py

---------

Co-authored-by: Tom Augspurger <tom.augspurger88@gmail.com>

adlfs/spec.py

@@ -147,9 +147,9 @@ class AzureBlobFileSystem(AsyncFileSystem):
         request session. See
         http://azure.microsoft.com/en-us/documentation/articles/storage-configure-connection-string/
         for the connection string format.
-    credential: TokenCredential or SAS token
+    credential: azure.core.credentials_async.AsyncTokenCredential or SAS token
         The credentials with which to authenticate.  Optional if the account URL already has a SAS token.
-        Can include an instance of TokenCredential class from azure.identity
+        Can include an instance of TokenCredential class from azure.identity.aio.
     blocksize: int
         The block size to use for download/upload operations. Defaults to hardcoded value of
         ``BlockBlobService.MAX_BLOCK_SIZE``