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/net/ipv4/ipconfig.c b/net/ipv4/ipconfig.c
index 7c992fb..4824fe8 100644
--- a/net/ipv4/ipconfig.c
+++ b/net/ipv4/ipconfig.c
@@ -1411,7 +1411,7 @@
 
 /*
  *  Decode any IP configuration options in the "ip=" or "nfsaddrs=" kernel
- *  command line parameter.  See Documentation/nfsroot.txt.
+ *  command line parameter.  See Documentation/filesystems/nfsroot.txt.
  */
 static int __init ic_proto_name(char *name)
 {