(in|out)_channel_of_descr: improve documentation (Christophe Troestler)

Following a discussion on the mailing list, it appears that more
explanation on the closing of channels created from file descriptors
is desirable.

git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@15142 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02