gstrfuncs: Document the behaviour of g_strjoinv()
gstrfuncs: Document the behaviour of g_strjoinv()

The behaviour of g_strjoinv() isn't explicitely explained when the array
contains less than 2 items. This removes the guesswork.

https://bugzilla.gnome.org/show_bug.cgi?id=764092

johne53/MB3Glib,endlessm/glib,MathieuDuponchelle/glib,MathieuDuponchelle/glib,johne53/MB3Glib,endlessm/glib,johne53/MB3Glib,endlessm/glib,endlessm/glib,endlessm/glib,johne53/MB3Glib,johne53/MB3Glib,MathieuDuponchelle/glib,johne53/MB3Glib,MathieuDuponchelle/glib,MathieuDuponchelle/glib