drm/vblank: Add intro to documentation

Lyude Paul wrote a very good intro to vblank here:
https://lore.kernel.org/dri-devel/[email protected]/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  - Add historical reference for blanking (Alex)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 <[email protected]>
Co-developed-by: Lyude Paul <[email protected]>
Reviewed-by: Lyude Paul <[email protected]>
Reviewed-by: Alex Deucher <[email protected]>
Cc: Alex Deucher <[email protected]>
Cc: Liviu Dudau <[email protected]>
Cc: Lyude Paul <[email protected]>
Cc: Thomas Zimmermann <[email protected]>
Cc: Daniel Vetter <[email protected]>
Cc: Maarten Lankhorst <[email protected]>
Cc: Maxime Ripard <[email protected]>
Cc: David Airlie <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]

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