[media] v4l: Document explicit synchronization

[media] v4l: Document explicit synchronization

Add section to VIDIOC_QBUF and VIDIOC_QUERY_BUF about it

v6:	- Close some gaps in the docs (Hans)

v5:
	- Remove V4L2_CAP_ORDERED
	- Add doc about V4L2_FMT_FLAG_UNORDERED

v4:
	- Document ordering behavior for in-fences
	- Document V4L2_CAP_ORDERED capability
	- Remove doc about OUT_FENCE event
	- Document immediate return of out-fence in QBUF

v3:
	- make the out_fence refer to the current buffer (Hans)
	- Note what happens when the IN_FENCE is not set (Hans)

v2:
	- mention that fences are files (Hans)
	- rework for the new API

Change-Id: I011c3e013caed2431c330eed1afeb8f52fe52ddc
Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.com>
Signed-off-by: Janghyuck Kim <janghyuck.kim@samsung.com>