man: drop documentation of internal lookup flags

man: drop documentation of internal lookup flags

This removes documentation of SD_RESOLVED_REQUIRE_PRIMARY and
SD_RESOLVED_CLAMP_TTL, which are internal flags, and not usable from
outside of resolved. They are refused by D-Bus APIs, for a reason.

Various other fixes/clean-ups of the relevant docs (including reordering
of the flags docs by "grouping" rather than bit values).

Fixes: #26619