docs: Improve row selection docs (#5184)

* Improve row selection docs

* Added note about row selection being index based by default
* Added a tanstack link for how row selection can managed with a custom id using the `getRowId` option on the core table api.
* Referenced the use case of manual pagination as it is one of the most likely scenarios that using a custom id is necessary for row selection

This should help developers understand how row selection is implemented.

* reword

---------

Co-authored-by: Kevin Van Cott <kevinvandy656@gmail.com>

Author: setfloat

docs/api/features/row-selection.md

@@ -15,6 +15,8 @@ export type RowSelectionTableState = {
 }
 ```
 
+By default, the row selection state uses the index of each row as the row identifiers. Row selection state can instead be tracked with a custom unique row id by passing in a custom [getRowId](../../api/core/table.md#getrowid) function to the the table.
+
 ## Table Options
 
 ### `enableRowSelection`