Merge pull request #17386 from vjpai/ctx_lifetime

Document that ClientContext must remain alive for duration of RPC

include/grpcpp/impl/codegen/client_context.h

@@ -168,6 +168,8 @@ class InteropClientContextInspector;
 /// (see \a grpc::CreateCustomChannel).
 ///
 /// \warning ClientContext instances should \em not be reused across rpcs.
+/// \warning The ClientContext instance used for creating an rpc must remain
+///          alive and valid for the lifetime of the rpc.
 class ClientContext {
  public:
   ClientContext();