Add documentation for GET snapshots state query parameter.

[JeremyDahlgren]

Adds documentation for the SnapshotState enum and state query parameter added in elasticsearch PR 128635.

[github-actions]

Following you can find the validation results for the APIs you have changed.

API	Status	Request	Response
snapshot.cleanup_repository	🟢	3/3	3/3
snapshot.clone	🟢	5/5	5/5
snapshot.create_repository	🔴	24/29	29/29
snapshot.create	🟢	30/30	30/30
snapshot.delete_repository	🟢	10/10	10/10
snapshot.delete	🔴	22/24	24/24
snapshot.get_repository	🔴	19/19	7/19
snapshot.get	🔴	15/16	16/16
snapshot.repository_analyze	⚪	Missing test	Missing test
snapshot.repository_verify_integrity	⚪	Missing test	Missing test
snapshot.restore	🟢	5/5	5/5
snapshot.status	🟢	2/2	2/2
snapshot.verify_repository	🟢	2/2	2/2

You can validate these APIs yourself by using the make validate target.