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.

Sp

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

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