Add warning re :prefix-map in map! docstring
Add warning re :prefix-map in map! docstring

It really shouldn't be used outside of Doom proper; it has the
capability of destroying other :prefix-map's due to variable/keymap
naming collisions.

hlissner/doom-emacs,hlissner/doom-emacs,UndeadKernel/emacs_doom,UndeadKernel/emacs_doom,UndeadKernel/emacs_doom,hlissner/.emacs.d,UndeadKernel/emacs_doom,hlissner/doom-emacs,UndeadKernel/emacs_doom,hlissner/.emacs.d,hlissner/doom-emacs,hlissner/doom-emacs,UndeadKernel/emacs_doom,hlissner/.emacs.d,hlissner/.emacs.d,hlissner/doom-emacs,hlissner/doom-emacs,hlissner/.emacs.d,hlissner/.emacs.d,UndeadKernel/emacs_doom,hlissner/.emacs.d,hlissner/.emacs.d