Improve the documentation about newly added exception classes

Author: lidizheng

src/python/grpcio/grpc/_cython/_cygrpc/aio/common.pyx.pxi

@@ -75,11 +75,16 @@ _COMPRESSION_METADATA_STRING_MAPPING = {
 }
 
 class BaseError(Exception):
-    """The base class for all exceptions generated by gRPC framework."""
+    """The base class for exceptions generated by gRPC AsyncIO stack."""
 
 
 class UsageError(BaseError):
-    """Raised when the usage might lead to undefined behavior."""
+    """Raised when the usage of API by applications is inappropriate.
+
+    For example, trying to invoke RPC on a closed channel, mixing two styles
+    of streaming API on the client side. This exception should not be
+    suppressed.
+    """
 
 
 class AbortError(BaseError):
@@ -91,4 +96,4 @@ class AbortError(BaseError):
 
 
 class InternalError(BaseError):
-    """Raised when unexpected error returned by Core."""
+    """Raised upon unexpected errors in native code."""

src/python/grpcio/grpc/experimental/aio/_call.py

@@ -443,7 +443,10 @@ async def write(self, request: RequestType) -> None:
         await self._write(request)
 
     async def done_writing(self) -> None:
-        """Implementation of done_writing is idempotent."""
+        """Signal peer that client is done writing.
+
+        This method is idempotent.
+        """
         self._raise_for_different_style(_APIStyle.READER_WRITER)
         await self._done_writing()