[Docs] Store Docs fixes about HashStore API (#47643)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/47643
Updating the docs to indicate the `num_keys` and `delete_key` APIs are now supported by the HashStore (not just TCPStore).
ghstack-source-id: 116459958
Test Plan: CI
Reviewed By: jiayisuse, mrshenli
Differential Revision: D24633570
fbshipit-source-id: 549479dd99f9ec6decbfffcb74b9792403d05ba2
