drm/fourcc: Add DOC: overview comment

There's a number of things which haven't previously been documented
around the usage of format modifiers. Capture the current
understanding in an overview comment and add it to the rst
documentation.

Ideally, the generated documentation would also include documentation
of all of the #defines, but the kernel-doc system doesn't currently
support kernel-doc comments on #define constants.

Suggested-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Brian Starkey <brian.starkey@arm.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Alexandru Gheorghe <alexandru-cosmin.gheorghe@arm.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180821161611.10424-1-brian.starkey@arm.com