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.com
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/filesystems/nfs/nfsd-admin-interfaces.txt

@@ -1,41 +0,0 @@
-Administrative interfaces for nfsd
-^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-
-Note that normally these interfaces are used only by the utilities in
-nfs-utils.
-
-nfsd is controlled mainly by pseudofiles under the "nfsd" filesystem,
-which is normally mounted at /proc/fs/nfsd/.
-
-The server is always started by the first write of a nonzero value to
-nfsd/threads.
-
-Before doing that, NFSD can be told which sockets to listen on by
-writing to nfsd/portlist; that write may be:
-
-	- an ascii-encoded file descriptor, which should refer to a
-	  bound (and listening, for tcp) socket, or
-	- "transportname port", where transportname is currently either
-	  "udp", "tcp", or "rdma".
-
-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.
-
-Between startup and shutdown, the number of threads may be adjusted up
-or down by additional writes to nfsd/threads or by writes to
-nfsd/pool_threads.
-
-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.
-For nfsd this is done using nfsd_mutex.