The rdev pointer kept in the local 'config' for each for
raid1, raid10, raid4/5/6 has non-obvious lifetime rules.
Sometimes RCU is needed, sometimes a lock, something nothing.

Add documentation to explain this.
Signed-off-by: NNeilBrown <neilb@suse.com>
Signed-off-by: NShaohua Li <sh.li@alibaba-inc.com>