/search_users admin API is undocumented

This issue has been migrated from [#16443](https://github.com/matrix-org/synapse/issues/16443).

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I'm not sure if this was done on purpose, but if so we should note that and deprecate it:

https://github.com/matrix-org/synapse/blob/95dc68eb46ebf7106575a0d4d25dcb7ebf9234ef/synapse/rest/admin/users.py#L805-L856