docs: fs: convert docs without extension to ReST

There are 3 remaining files without an extension inside the fs docs
dir.

Manually convert them to ReST.

In the case of the nfs/exporting.rst file, as the nfs docs
aren't ported yet, I opted to convert and add a :orphan: there,
with should be removed when it gets added into a nfs-specific
part of the fs documentation.

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
diff --git a/fs/cifs/export.c b/fs/cifs/export.c
index ce8b7f6..eb0bb8c 100644
--- a/fs/cifs/export.c
+++ b/fs/cifs/export.c
@@ -24,7 +24,7 @@
  */
 
  /*
-  * See Documentation/filesystems/nfs/Exporting
+  * See Documentation/filesystems/nfs/exporting.rst
   * and examples in fs/exportfs
   *
   * Since cifs is a network file system, an "fsid" must be included for