docs(useQuery): explain what an identity function is for placeholderData (#6075)

* docs(useQuery): explain identity function for placeholderData

clarify the usage of an identity function when used with placeholderData

* docs(useQuery): remove the word exported

remove the word exported regarding keepPreviousData

Author: SeanCassiere

docs/react/guides/migrating-to-v5.md

@@ -234,12 +234,16 @@ Since the only supported syntax now is the object syntax, this rule is no longer
 
 We have removed the `keepPreviousData` option and `isPreviousData` flag as they were doing mostly the same thing as `placeholderData` and `isPlaceholderData` flag.
 
-To achieve the same functionality as `keepPreviousData`, we have added previous query `data` as an argument to `placeholderData` function.
-Therefore you just need to provide an identity function to `placeholderData` or use `keepPreviousData` function returned from Tanstack Query.
+To achieve the same functionality as `keepPreviousData`, we have added previous query `data` as an argument to `placeholderData` which accepts an identity function. Therefore you just need to provide an identity function to `placeholderData` or use the included `keepPreviousData` function from Tanstack Query.
 
 > A note here is that `useQueries` would not receive `previousData` in the `placeholderData` function as argument. This is due to a dynamic nature of queries passed in the array, which may lead to a different shape of result from placeholder and queryFn.
 
 ```diff
+import {
+   useQuery,
++  keepPreviousData
+} from "@tanstack/react-query";
+
 const {
    data,
 -  isPreviousData,
@@ -252,6 +256,16 @@ const {
 });
 ```
 
+An identity function, in the context of Tanstack Query, refers to a function that always returns its provided argument (i.e. data) unchanged.
+
+```ts
+useQuery({
+  queryKey,
+  queryFn,
+  placeholderData: (previousData, previousQuery) => previousData, // identity function with the same behaviour as `keepPreviousData`
+})
+```
+
 There are some caveats to this change however, which you must be aware of:
 
 - `placeholderData` will always put you into `success` state, while `keepPreviousData` gave you the status of the previous query. That status could be `error` if we have data fetched successfully and then got a background refetch error. However, the error itself was not shared, so we decided to stick with behavior of `placeholderData`.