doc/rgw: warn about rgw_usage_max_shards consistency

doc/rgw: warn about rgw_usage_max_shards consistency

Add documentation warnings explaining that all RGW daemons and
radosgw-admin commands must use the same rgw_usage_max_shards value.
Mismatched shard counts cause writes and reads/trim to target different
objects, resulting in seemingly empty usage logs or failed cleanup.

Also document the --rgw-usage-max-shards command-line parameter for
radosgw-admin as an alternative to global config.

Fixes: https://tracker.ceph.com/issues/76459
Signed-off-by: Olivier Chaze <olivier.chaze@infomaniak.com>