drm/doc: document that PRIME import/export is always supported

Since commit 6b85aa68 ("drm: Enable PRIME import/export for all
drivers"), import/export is always supported. Document this so that
user-space knows what to expect.

Signed-off-by: Simon Ser <contact@emersion.fr>
Reviewed-by: Jeffrey Hugo <quic_jhugo@quicinc.com>
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: Jeffrey Hugo <quic_jhugo@quicinc.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20230712183156.191445-1-contact@emersion.fr