gmarkup: make documentation more discoverable

There is a bunch of documentation in a separate markdown page that does
not appear in search results. We should point to it.

I think the restriction on not being used to process untrusted input
should be relaxed into something more permissive, like "should not be
used on untrusted input if you care about denial of service." But that
can be a problem for another day.

glib/gmarkup.h

@@ -143,6 +143,10 @@ typedef struct _GMarkupParser GMarkupParser;
  * errors are intended to be set from these callbacks. If you set an error
  * from a callback, g_markup_parse_context_parse() will report that error
  * back to its caller.
+ *
+ * Refer to the [GMarkup](../glib/markup.html) documentation to understand
+ * the scope and limitations of `GMarkupParser`. In particular, it is not a
+ * full XML parser and it must not be used to process untrusted data.
  */
 struct _GMarkupParser
 {