drm/vblank: Add intro to documentation

drm/vblank: Add intro to documentation

Lyude Paul wrote a very good intro to vblank here:
https://lore.kernel.org/dri-devel/faf63d8a9ed23c16af69762f59d0dca6b2bf085f.camel@redhat.com/T/#mce6480be738160e9d07c5d023e88fd78d7a06d27

Add this to the intro chapter in drm_vblank.c so others
can benefit from it too.

v2:
  - Reworded to improve readability (Thomas)

v3:
  - Added nice ascii drawing from Lyude (Lyude)
  - Added referende to high-precision timestamp (Daniel)
  - Improved grammar (Thomas)
  - Combined it all and made kernel-doc happy
  - Dropped any a-b, r-b do to the amount of changes

v4:
  - Add intro to vblank interrupt (Liviu)
  - Add historical reference for blanking (Alex)

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Co-developed-by: Lyude Paul <lyude@redhat.com>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: Liviu Dudau <liviu.dudau@arm.com>
Cc: Lyude Paul <lyude@redhat.com>
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Maxime Ripard <mripard@kernel.org>
Cc: David Airlie <airlied@linux.ie>
Link: https://patchwork.freedesktop.org/patch/msgid/20200406194746.26433-2-sam@ravnborg.org