maintenance: Document secondary purpose of --server

maintenance: Document secondary purpose of --server

Follows-up to 2de3bf45d12d8. That commit references "Wikimedia Method"
of https://www.mediawiki.org/wiki/Manual:Wiki_family. This is slightly
confusing given that page explicitly states that hostname-based detection
is only used on the web server, with code for MW_DB and --wiki in place.

Nonetheless, despite this being seemingly unused, unadvertised
and undocumented, if we're going to support this, we might as well
document what its for, to avoid it being removed without notice in
the future.

Change-Id: I9ff333b6e9f15cfe4de21ef01a47931dd73d297e