libubi: ubi_update_start: slightly improve documentation

Calling ubi_update_start() may fail if exclusive access to the UBI
volume cannot be established by the kernel. This is likely to happen if
an UBI volume is updated directly after creation, because current
versions of (systemd-)udevd run their internal 'blkid' also for UBI
volumes as soon as they appear (trying to read the UBIFS UUID).

There are mainly to options for this case:
1. Don't allow udevd to read UBI volumes at all (means loosing
functionatlity)
2. Wait until udevd has finished accessing a new volume (requires
cooperation with udevd)
3. Simply retry after short time in case of EBUSY

For using option 3, the documentation should state that the concrete
error code can be read via errno (this has always been the case since
libubi has been introduced).

Link: https://groups.google.com/g/swupdate/c/8NVooKjD9oo
Signed-off-by: Christian Eggers <ceggers@arri.de>
Reviewed-by: Zhihao Cheng <chengzhihao1@huawei.com>
Signed-off-by: David Oberhollenzer <david.oberhollenzer@sigma-star.at>

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