9p: document short read behaviour with O_NONBLOCK

Regular files opened with O_NONBLOCK allow read to return after a single
round-trip with the server instead of trying to fill buffer.
Add a few lines in 9p documentation to describe that.

Link: http://lkml.kernel.org/r/1586193572-1375-1-git-send-email-asmadeus@codewreck.org


Signed-off-by: Dominique Martinet <dominique.martinet@cea.fr>