NFSv4.1: lseg documentation

Signed-off-by: NFred Isaman <iisaman@netapp.com>
Signed-off-by: NTrond Myklebust <Trond.Myklebust@netapp.com>

Documentation/filesystems/nfs/pnfs.txt

@@ -46,3 +46,10 @@ data server cache
 file driver devices refer to data servers, which are kept in a module
 level cache.  Its reference is held over the lifetime of the deviceid
 pointing to it.
+
+lseg
+----
+lseg maintains an extra reference corresponding to the NFS_LSEG_VALID
+bit which holds it in the pnfs_layout_hdr's list.  When the final lseg
+is removed from the pnfs_layout_hdr's list, the NFS_LAYOUT_DESTROYED
+bit is set, preventing any new lsegs from being added.