docs(useMutation): clarify mutationFn option default (#4837)

* docs(useMutation): clarify `mutationFn` option default

* Make optionality consistent with `useQuery` documentation

Co-authored-by: Dominik Dorfmeister <office@dorfmeister.cc>

Co-authored-by: Dominik Dorfmeister <office@dorfmeister.cc>

docs/react/reference/useMutation.md

@@ -43,7 +43,7 @@ mutate(variables, {
 **Options**
 
 - `mutationFn: (variables: TVariables) => Promise<TData>`
-  - **Required**
+  - **Required, but only if no default mutation function has been defined**
   - A function that performs an asynchronous task and returns a promise.
   - `variables` is an object that `mutate` will pass to your `mutationFn`
 - `cacheTime: number | Infinity`