ustring: Be extra clear about "count/types/order"
ustring: Be extra clear about "count/types/order"

Say the same thing both in the main blurb and the argument documentation
– and say "count" instead of "number" just so it's extra clear what I
mean (and because "size" didn't seem clear enough in that sense).

GNOME/glibmm,GNOME/glibmm,GNOME/glibmm,GNOME/glibmm,GNOME/glibmm