freedreno/a6xx: Add some documentation for shared consts

I'm not convinced we'll actually want to use this, and there may be
another enable bit in SP_UNKNOWN_AB00, but it's nice to at least write
this down in case we want to try using it in the future.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5877>
3 files changed