drm/bridge: document that adding a bridge is mandatory before attach

drm/bridge: document that adding a bridge is mandatory before attach

At the moment it's not documented that you need to add a bridge before
attaching it. Clarify that.

Suggested-by: Maxime Ripard <mripard@kernel.org>
Link: https://lore.kernel.org/all/20250709-sophisticated-loon-of-rain-6ccdd8@houat/
Reviewed-by: Raphael Gallais-Pou <rgallaispou@gmail.com>
Reviewed-by: Maxime Ripard <mripard@kernel.org>
Link: https://patch.msgid.link/20251028-b4-drm-bridge-alloc-add-before-attach-v3-3-bb8611acbbfb@bootlin.com
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>