dma-fence: Document recoverable page fault implications

Recently there was a fairly long thread about recoreable hardware page
faults, how they can deadlock, and what to do about that.

While the discussion is still fresh I figured good time to try and
document the conclusions a bit. This documentation section explains
what's the potential problem, and the remedies we've discussed,
roughly ordered from best to worst.

v2: Linus -> Linux typoe (Dave)

v3:
- Make it clear drivers only need to implement one option (Christian)
- Make it clearer that implicit sync is out the window with exclusive
  fences (Christian)
- Add the fairly theoretical option of segementing the memory (either
  statically or through dynamic checks at runtime for which piece of
  memory is managed how) and explain why it's not a great idea (Felix)

References: https://lore.kernel.org/dri-devel/[email protected]/
Reviewed-by: Christian König <[email protected]>
Acked-by: Thomas Hellström <[email protected]>
Reviewed-by: Felix Kuehling <[email protected]>
c: Dave Airlie <[email protected]>
Cc: Maarten Lankhorst <[email protected]>
Cc: Thomas Hellström <[email protected]>
Cc: "Christian König" <[email protected]>
Cc: Jerome Glisse <[email protected]>
Cc: Felix Kuehling <[email protected]>
Signed-off-by: Daniel Vetter <[email protected]>
Cc: Sumit Semwal <[email protected]>
Cc: [email protected]
Cc: [email protected]
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]

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