xarray.h: correct return code documentation for xa_store_{bh,irq}()

__xa_store() and xa_store() document that the functions can fail, and
that the return code can be an xa_err() encoded error code.

xa_store_bh() and xa_store_irq() do not document that the functions can
fail and that they can also return xa_err() encoded error codes.

Thus: Update the documentation.

Signed-off-by: Manfred Spraul <manfred@colorfullife.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Matthew Wilcox <willy@infradead.org>
Link: http://lkml.kernel.org/r/20200430111424.16634-1-manfred@colorfullife.com
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>