1) Remove documentation of the groff-1.15 compatibility quirk

1) Remove documentation of the groff-1.15 compatibility quirk
of suppressing spacing before a third .Xr argument
because that quirk was removed in mdoc_macro.c rev. 1.113.

2) Mark the "section" argument to .Xr as (syntactically) optional,
but still do not encourage omitting it.
The missing .Op was reported by espie@.

Wording tweaked by and ok jmc@, ok millert@.