Documentation: move nfsroot.txt to filesystems/

Documentation/ is a little large, and filesystems/ seems an obvious
place for this file.

Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
diff --git a/fs/Kconfig b/fs/Kconfig
index d731282..c509123 100644
--- a/fs/Kconfig
+++ b/fs/Kconfig
@@ -1744,10 +1744,10 @@
 	  If you want your Linux box to mount its whole root file system (the
 	  one containing the directory /) from some other computer over the
 	  net via NFS (presumably because your box doesn't have a hard disk),
-	  say Y. Read <file:Documentation/nfsroot.txt> for details. It is
-	  likely that in this case, you also want to say Y to "Kernel level IP
-	  autoconfiguration" so that your box can discover its network address
-	  at boot time.
+	  say Y. Read <file:Documentation/filesystems/nfsroot.txt> for
+	  details. It is likely that in this case, you also want to say Y to
+	  "Kernel level IP autoconfiguration" so that your box can discover
+	  its network address at boot time.
 
 	  Most people say N here.