dma-resv: Give the docs a do-over

Specifically document the new/clarified rules around how the shared
fences do not have any ordering requirements against the exclusive
fence.

But also document all the things a bit better, given how central
struct dma_resv to dynamic buffer management the docs have been very
inadequat.

- Lots more links to other pieces of the puzzle. Unfortunately
  ttm_buffer_object has no docs, so no links :-(

- Explain/complain a bit about dma_resv_locking_ctx(). I still don't
  like that one, but fixing the ttm call chains is going to be
  horrible. Plus we want to plug in real slowpath locking when we do
  that anyway.

- Main part of the patch is some actual docs for struct dma_resv.

Overall I think we still have a lot of bad naming in this area (e.g.
dma_resv.fence is singular, but contains the multiple shared fences),
but I think that's more indicative of how the semantics and rules are
just not great.

Another thing that's real awkard is how chaining exclusive fences
right now means direct dma_resv.exclusive_fence pointer access with an
rcu_assign_pointer. Not so great either.

v2:
- Fix a pile of typos (Matt, Jason)
- Hammer it in that breaking the rules leads to use-after-free issues
  around dma-buf sharing (Christian)

Reviewed-by: Christian König <[email protected]>
Cc: Jason Ekstrand <[email protected]>
Cc: Matthew Auld <[email protected]>
Reviewed-by: Matthew Auld <[email protected]>
Signed-off-by: Daniel Vetter <[email protected]>
Cc: Sumit Semwal <[email protected]>
Cc: "Christian König" <[email protected]>
Cc: [email protected]
Cc: [email protected]
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]

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