The gtk-doc text that the tool receives is not XML, it's a plain text.
When setting the plain text as XML attribute, we need to properly escape
it. The previous XML escape code was naive, and didn't cover for a
plain ampersand.

[thaller@redhat.com: adjusted patch during backport from nm-1-26 to nm-1-24]

(cherry picked from commit 1641cc1d)
(cherry picked from commit 5b7d39f8)
(cherry picked from commit 5c50280b)