Clarify cancel before call is started

src/objective-c/GRPCClient/GRPCCall.h

@@ -245,6 +245,7 @@ extern NSString *const kGRPCTrailersKey;
 
 /**
  * Starts the call. This function should only be called once; additional calls will be discarded.
+ * Invokes after calling cancel: are discarded.
  */
 - (void)start;
 

src/objective-c/GRPCClient/GRPCCall.m

@@ -209,6 +209,7 @@ const char *kCFStreamVarName = "grpc_cfstream";
 
 - (void)cancel {
   dispatch_async(_dispatchQueue, ^{
+    self->_started = YES;
     if (self->_call) {
       [self->_call cancel];
       self->_call = nil;