argus on #775 v3: "the `gatewayHost` option description's
server_name-vs-gatewayHost essay + federation SRV note are also
candidates for [docs/gateway.md] section."
Cuts the gatewayHost option's description from ~40 lines (with
inline duplication of the discovery flow, when-to-set-which, and
federation port caveat) down to ~8 lines pointing at
`docs/gateway.md`. The brief `server_name vs gatewayHost` clarifier
stays in code because it disambiguates two SIMILAR-LOOKING options
on the same module — operators reading option docs need the
distinction inline, not behind a doc link.
Also trimmed `matrix.gui.enable` + `matrix.gui.package` descriptions
to similar shapes — point at docs/gateway.md for the architecture,
keep the override-shape hints in code.
Push includes the rebase onto current main (#764 + 0af6ea1 + others
landed since #775 was opened; cherry-picked commits get skipped
cleanly).
Net: matrix.nix loses ~70 lines of inline prose. No behavioral
change (verified gatewayHost still resolves to `matrix.<hive>`).
8d25b087b6