gdbusconnection: Clarify nullability in a documentation comment
gdbusconnection: Clarify nullability in a documentation comment

The comment previously said ‘never %NULL’, but it wasn’t clear whether
this meant `(not nullable)` or `(not optional)`. From looking at the
code, it means `(not optional)`.

Clarify things by removing the prose. The annotations themselves should
be clear and explicit enough.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

Closes: #1836

endlessm/glib,endlessm/glib,endlessm/glib,endlessm/glib,endlessm/glib