manpage: input: document script_message command
manpage: lua: fix typo
manpage: lua: improve introduction, add simplistic example
manpage: lua: attempt to improve add_key_binding() description
manpage: lua: shorten description of register_script_command()

This is simply not important enough to warrant so much space, and it's
perhaps also very confusing.

Although I'm not fully sure, since this is about the only way that
allows a user to interact with a script, besides key bindings and static
options.
vo_opengl: use GL_RGBA16 FBO format for HQ

GL_RGB16 is definitely not a required format, after review of the
OpenGL 3.3 spec. Most HW doesn't have native support for RGB16 anyway.