docs/features.txt: VK_EXT_separate_stencil_usage not exposed on RADV

Signed-off-by: Anthoine Bourgeois <anthoine.bourgeois@gmail.com>
Reported-by: Rhys Perry <pendingchaos02@gmail.com>
Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Fixes: 3d58ab7576d ("docs/features: Minor update extensions support")
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7466>
1 file changed