Update keymap docs to match current behaviour
Update keymap docs to match current behaviour

An overlooked API change in the gdk-backend work: many of the
keymap functions used to accept NULL to mean 'default keymap'.
They no longer do, so update the docs to match the new behaviour.

Adamovskiy/gtk,davidgumberg/gtk,simokivimaki/gtk,ahodesuka/gtk,jessevdk/gtk,chergert/gtk,ahodesuka/gtk,simokivimaki/gtk,Adamovskiy/gtk,Sidnioulz/SandboxGtk,jessevdk/gtk,Lyude/gtk-,jessevdk/gtk,chergert/gtk,msteinert/gtk,Adamovskiy/gtk,alexlarsson/gtk,ebassi/gtk,grubersjoe/adwaita,ahodesuka/gtk,chergert/gtk,davidgumberg/gtk,Sidnioulz/SandboxGtk,jigpu/gtk,Distrotech/gtk2,Distrotech/gtk2,ebassi/gtk,Lyude/gtk-,Lyude/gtk-,grubersjoe/adwaita,jigpu/gtk,Adamovskiy/gtk,jigpu/gtk,Distrotech/gtk2,Adamovskiy/gtk,msteinert/gtk,Adamovskiy/gtk,Lyude/gtk-,davidt/gtk,alexlarsson/gtk,alexlarsson/gtk,Sidnioulz/SandboxGtk,ebassi/gtk,Adamovskiy/gtk,davidgumberg/gtk,ahodesuka/gtk,jadahl/gtk,bratsche/gtk-,Lyude/gtk-,davidgumberg/gtk,alexlarsson/gtk,chergert/gtk,jigpu/gtk,jessevdk/gtk,chergert/gtk,davidt/gtk,ahodesuka/gtk,bratsche/gtk-,jigpu/gtk,Lyude/gtk-,bratsche/gtk-,jigpu/gtk,msteinert/gtk,alexlarsson/gtk,bratsche/gtk-,grubersjoe/adwaita,Sidnioulz/SandboxGtk,jadahl/gtk,davidt/gtk,grubersjoe/adwaita,davidt/gtk,Sidnioulz/SandboxGtk,msteinert/gtk,Adamovskiy/gtk,alexlarsson/gtk,ebassi/gtk,davidgumberg/gtk,simokivimaki/gtk,jadahl/gtk,jessevdk/gtk,grubersjoe/adwaita,jigpu/gtk,ahodesuka/gtk,bratsche/gtk-,grubersjoe/adwaita,jigpu/gtk,ebassi/gtk,Distrotech/gtk2,davidt/gtk,jadahl/gtk,Sidnioulz/SandboxGtk,Lyude/gtk-,chergert/gtk,Distrotech/gtk2,Lyude/gtk-,davidgumberg/gtk,msteinert/gtk,jadahl/gtk,jessevdk/gtk,davidt/gtk,simokivimaki/gtk,chergert/gtk,jadahl/gtk,ahodesuka/gtk,alexlarsson/gtk,msteinert/gtk,ahodesuka/gtk,jadahl/gtk,ebassi/gtk,grubersjoe/adwaita,alexlarsson/gtk,jadahl/gtk,Distrotech/gtk2,simokivimaki/gtk,simokivimaki/gtk,davidgumberg/gtk,chergert/gtk,davidgumberg/gtk,jessevdk/gtk,bratsche/gtk-,grubersjoe/adwaita