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/exportfs/expfs.c b/fs/exportfs/expfs.c
index f0e5497..09bc687 100644
--- a/fs/exportfs/expfs.c
+++ b/fs/exportfs/expfs.c
@@ -7,7 +7,7 @@
  * and for mapping back from file handles to dentries.
  *
  * For details on why we do all the strange and hairy things in here
- * take a look at Documentation/filesystems/nfs/Exporting.
+ * take a look at Documentation/filesystems/nfs/exporting.rst.
  */
 #include <linux/exportfs.h>
 #include <linux/fs.h>