Documentation: convert nfsd-admin-interfaces to ReST

Convert nfsd-admin-interfaces to ReST and move it into admin-guide.
Content remains mostly untouched.
Signed-off-by: Daniel W. S. Almeida <dwlsalmeida@gmail.com>
Link: https://lore.kernel.org/r/d471305e9c96dec38f18d2ff816fca2269a88e29.1578697871.git.dwlsalmeida@gmail.comSigned-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/admin-guide/nfs/index.rst

@@ -8,3 +8,4 @@ NFS
     nfs-client
     nfsroot
     nfs-rdma
+    nfsd-admin-interfaces

Documentation/filesystems/nfs/nfsd-admin-interfaces.txt → Documentation/admin-guide/nfs/nfsd-admin-interfaces.rst

+==================================
 Administrative interfaces for nfsd
-^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+==================================
 
 Note that normally these interfaces are used only by the utilities in
 nfs-utils.
@@ -21,10 +22,8 @@ writing to nfsd/portlist; that write may be:
 If nfsd is started without doing any of these, then it will create one
 udp and one tcp listener at port 2049 (see nfsd_init_socks).
 
-On startup, nfsd and lockd grace periods start.
-
-nfsd is shut down by a write of 0 to nfsd/threads.  All locks and state
-are thrown away at that point.
+On startup, nfsd and lockd grace periods start. nfsd is shut down by a write of
+0 to nfsd/threads.  All locks and state are thrown away at that point.
 
 Between startup and shutdown, the number of threads may be adjusted up
 or down by additional writes to nfsd/threads or by writes to
@@ -34,7 +33,7 @@ For more detail about files under nfsd/ and what they control, see
 fs/nfsd/nfsctl.c; most of them have detailed comments.
 
 Implementation notes
-^^^^^^^^^^^^^^^^^^^^
+====================
 
 Note that the rpc server requires the caller to serialize addition and
 removal of listening sockets, and startup and shutdown of the server.