This are nasty, because they mean words get dropped from the
documentation.  This can be seen at

 - https://docs.gtk.org/gio/ctor.DBusNodeInfo.new_for_xml.html where
   it reads "one top-level element" instead of "one top-level <node>
   element".

 - https://docs.gtk.org/gio/method.ProxyResolver.lookup.html where it
   reads "where could be" instead of "where <protocol> could be".

 - https://docs.gtk.org/gio/method.Socket.get_option.html where it
   reads "[][gio-gnetworking.h]" instead of
   "[<gio/gnetworking.h>][gio-gnetworking.h" (also, this markdown link
   needs fixed, but let's save that for another commit).

 - https://docs.gtk.org/glib/ctor.DateTime.new_from_iso8601.html where
   the text is incomprehensible; "strings of the form are supported"
   instead of "strings of the form <date><sep><time><tz> are
   supported"; further references to <sep>, <date>, <time>, and <tz>
   are similarly mangled.

 - https://docs.gtk.org/glib/method.MatchInfo.fetch_named.html and
   https://docs.gtk.org/glib/method.MatchInfo.fetch_named_pos.html
   where the regex reads as "(?Pa)?b" instead of as "(?P<X>a)?b",
   changing the meaning of it.

 - https://docs.gtk.org/glib/method.Regex.match_all_full.html is all
   wack because the "<a>" in the example string is taken to be an HTML
   link; and all example strings and regexes are mangled (also, one of
   the regexes has a stray ";" in it, but let's save that for another
   commit).

 - https://docs.gtk.org/glib/method.Regex.replace.html where it simply
   reads "\g" instead of "\g<number>" and "\g<name>".

Fix those.