docs(queries):add description of isFetching (#5887)

Author: a-yoshizawa-mediba

docs/react/guides/queries.md

@@ -48,6 +48,7 @@ Beyond those primary states, more information is available depending on the stat
 
 - `error` - If the query is in an `isError` state, the error is available via the `error` property.
 - `data` - If the query is in an `isSuccess` state, the data is available via the `data` property.
+- `isFetching` - In any state, if the query is fetching at any time (including background refetching) `isFetching` will be `true`.
 
 For **most** queries, it's usually sufficient to check for the `isLoading` state, then the `isError` state, then finally, assume that the data is available and render the successful state: