lib/ida: document locking requirements a bit better

I wanted to wrap a bunch of ida_simple_get calls into their own locking,
until I dug around and read the original commit message.  Stuff like
this should imo be added to the kernel doc, let's do that.

Link: http://lkml.kernel.org/r/20161027072216.20411-1-daniel.vetter@ffwll.ch


Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Acked-by: Tejun Heo <tj@kernel.org>
Cc: Mel Gorman <mgorman@techsingularity.net>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>