[doc] Clarify client callback API comments (#37805)

Comments say users "should not" call Start* methods after notification callbacks are called with false, but users may still call so it's close to 'not recommended'. But it's less confusing to remove it altogether.

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Closes #37805

COPYBARA_INTEGRATE_REVIEW=https://github.com/grpc/grpc/pull/37805 from yousukseung:client_callback_doc_clarify 38b9371f79
PiperOrigin-RevId: 679709417

include/grpcpp/support/client_callback.h

@@ -325,21 +325,20 @@ class ClientBidiReactor : public internal::ClientReactor {
   /// call of OnReadDone or OnDone.
   ///
   /// \param[in] ok Was the initial metadata read successfully? If false, no
-  ///               new read/write operation will succeed, and any further
-  ///               Start* operations should not be called.
+  ///               new read/write operation will succeed.
   virtual void OnReadInitialMetadataDone(bool /*ok*/) {}
 
   /// Notifies the application that a StartRead operation completed.
   ///
   /// \param[in] ok Was it successful? If false, no new read/write operation
-  ///               will succeed, and any further Start* should not be called.
+  ///               will succeed.
   virtual void OnReadDone(bool /*ok*/) {}
 
   /// Notifies the application that a StartWrite or StartWriteLast operation
   /// completed.
   ///
   /// \param[in] ok Was it successful? If false, no new read/write operation
-  ///               will succeed, and any further Start* should not be called.
+  ///               will succeed.
   virtual void OnWriteDone(bool /*ok*/) {}
 
   /// Notifies the application that a StartWritesDone operation completed. Note