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.
Part of !537. https://gitlab.freedesktop.org/NetworkManager/NetworkManager/-/merge_requests/537
They are not only used in "libnm/" directory. Move to "tools/".