drm/doc: Document ioctl errno value patterns

We're not super-consistent about these, but I think it's worth to
document at least the commmon patterns.

v2:
- Add a not about ENOTTY (it's just a confusing name, but used
exactly what it's meant for in DRM) (Chris).
- Unconfuse the text for ENODEV (Daniel)
- Move text undert the IOCTL heading (Chris).
- typos

Cc: Daniel Stone <[email protected]>
Cc: Joonas Lahtinen <[email protected]>
Cc: Chris Wilson <[email protected]>
Cc: "Zhang, Tina" <[email protected]>
Reviewed-by: Chris Wilson <[email protected]>
Signed-off-by: Daniel Vetter <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]

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