dma-buf: Update docs for SYNC ioctl

Just a bit of wording polish plus mentioning that it can fail and must
be restarted.

Requested by Sumit.

v2: Fix them typos (Hans).

Cc: Chris Wilson <[e-mail]>
Cc: Tiago Vignatti <[e-mail]>
Cc: Stéphane Marchesin <[e-mail]>
Cc: David Herrmann <[e-mail]>
Cc: Sumit Semwal <[e-mail]>
Cc: Daniel Vetter <[e-mail]>
CC: [e-mail]
Cc: [e-mail]
Cc: [e-mail]
Cc: [e-mail]
Cc: [e-mail]
Cc: Hans Verkuil <[e-mail]>
Acked-by: Sumit Semwal <[e-mail]>
Acked-by: Hans Verkuil <[e-mail]>
Signed-off-by: Daniel Vetter <[e-mail]>

Documentation/dma-buf-sharing.txt

···
 
    No special interfaces, userspace simply calls mmap on the dma-buf fd, making
    sure that the cache synchronization ioctl (DMA_BUF_IOCTL_SYNC) is *always*
-   used when the access happens. This is discussed next paragraphs.
+   used when the access happens. Note that DMA_BUF_IOCTL_SYNC can fail with
+   -EAGAIN or -EINTR, in which case it must be restarted.
 
    Some systems might need some sort of cache coherency management e.g. when
    CPU and GPU domains are being accessed through dma-buf at the same time. To
···
        want (with the new data being consumed by the GPU or say scanout device)
      - munmap once you don't need the buffer any more
 
-    Therefore, for correctness and optimal performance, systems with the memory
-    cache shared by the GPU and CPU i.e. the "coherent" and also the
-    "incoherent" are always required to use SYNC_START and SYNC_END before and
-    after, respectively, when accessing the mapped address.
+    For correctness and optimal performance, it is always required to use
+    SYNC_START and SYNC_END before and after, respectively, when accessing the
+    mapped address. Userspace cannot rely on coherent access, even when there
+    are systems where it just works without calling these ioctls.
 
 2. Supporting existing mmap interfaces in importers
 

drivers/dma-buf/dma-buf.c

···
  * @dmabuf:	[in]	buffer to complete cpu access for.
  * @direction:	[in]	length of range for cpu access.
  *
- * This call must always succeed.
+ * Can return negative error values, returns 0 on success.
  */
 int dma_buf_end_cpu_access(struct dma_buf *dmabuf,
 			   enum dma_data_direction direction)