Add doc for Hash Field Expiration

[sundb]

see https://github.com/redis/redis/pull/13303

1. Add a new field expired_hash_fields for INFO STATS.
2. Add a new field hashes_with_expiry_fields for INFO KEYSPACE.
3. Add two new notification events hexpire and hpersist.

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[dwdougherty]

Hi Debing Sun. Thank you for making these changes. I'll get them added to my existing PR on the new docs repo.

This repo site is effectively dead and it will be archived very soon. All future PRs/issues need to be raised on the new repo site.

[sundb]

@dwdougherty OK, thanks.