docs(gnmi): remove async context manager references from client

Remove outdated async documentation from GNMIClient module:
- Remove async context manager from feature list
- Remove async usage example from class docstring

The client now only documents synchronous usage patterns.

src/gnmi/client.py

@@ -3,7 +3,6 @@ gNMI Client Wrapper for Fabric Orchestrator.
 
 This module provides a high-level gNMI client using pygnmi with:
 - Connection management
-- Async context manager support
 - Error handling with meaningful exceptions
 """
 
@@ -82,10 +81,6 @@ class GNMIClient:
     Provides high-level methods for gNMI operations with proper
     error handling and context manager support.
 
-    Usage:
-        async with GNMIClient(host="leaf1", port=6030, ...) as client:
-            caps = await client.capabilities()
-
     Or synchronously:
         with GNMIClient(host="leaf1", port=6030, ...) as client:
             caps = client.capabilities()