loader: Move gfx functions to gfx.lua.8

Now that the fb_* and term_* functions are available in the gfx table,
move the documentation to gfx.lua.8. Add information about backwards
compatibility.

Sponsored by: Netflix
Reviewed by: kevans
Differential Revision: https://reviews.freebsd.org/D43907

(cherry picked from commit a8f8c53761c4b0436c9d546ebe046003f1b3d88f)