Documentation: convert nfs.txt to ReST

This patch converts nfs.txt to RST. It also moves it to admin-guide.
The reason for moving it is because this document contains information
useful for system administrators, as noted on the following paragraph:

'The purpose of this document is to provide information on some of the
special features of the NFS client that can be configured by system
administrators'.
Signed-off-by: Daniel W. S. Almeida <dwlsalmeida@gmail.com>
Link: https://lore.kernel.org/r/cb9f2da2f2f6dd432b4cf9e05f79f74f4d54b6ab.1578697871.git.dwlsalmeida@gmail.comSigned-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/admin-guide/index.rst

@@ -76,6 +76,7 @@ configure specific aspects of kernel behavior to your liking.
    device-mapper/index
    efi-stub
    ext4
+   nfs/index
    gpio/index
    highuid
    hw_random

Documentation/admin-guide/nfs/index.rst

+=============
+NFS
+=============
+
+.. toctree::
+    :maxdepth: 1
+
+    nfs-client

Documentation/filesystems/nfs/nfs.txt → Documentation/admin-guide/nfs/nfs-client.rst

+==========
+NFS Client
+==========
 
 The NFS client
 ==============
@@ -59,10 +62,11 @@ The DNS resolver
 
 NFSv4 allows for one server to refer the NFS client to data that has been
 migrated onto another server by means of the special "fs_locations"
-attribute. See
-	http://tools.ietf.org/html/rfc3530#section-6
-and
-	http://tools.ietf.org/html/draft-ietf-nfsv4-referrals-00
+attribute. See `RFC3530 Section 6: Filesystem Migration and Replication`_ and
+`Implementation Guide for Referrals in NFSv4`_.
+
+.. _RFC3530 Section 6\: Filesystem Migration and Replication: http://tools.ietf.org/html/rfc3530#section-6
+.. _Implementation Guide for Referrals in NFSv4: http://tools.ietf.org/html/draft-ietf-nfsv4-referrals-00
 
 The fs_locations information can take the form of either an ip address and
 a path, or a DNS hostname and a path. The latter requires the NFS client to
@@ -94,7 +98,8 @@ Assuming that the user has the 'rpc_pipefs' filesystem mounted in the usual
        script, and <ttl> is the 'time to live' of this cache entry (in
        units of seconds).
 
-       Note: If <ip address> is invalid, say the string "0", then a negative
+       .. note::
+            If <ip address> is invalid, say the string "0", then a negative
             entry is created, which will cause the kernel to treat the hostname
             as having no valid DNS translation.
 
@@ -103,26 +108,27 @@ Assuming that the user has the 'rpc_pipefs' filesystem mounted in the usual
 
 A basic sample /sbin/nfs_cache_getent
 =====================================
-
-#!/bin/bash
-#
-ttl=600
-#
-cut=/usr/bin/cut
-getent=/usr/bin/getent
-rpc_pipefs=/var/lib/nfs/rpc_pipefs
-#
-die()
-{
+.. code-block:: sh
+
+    #!/bin/bash
+    #
+    ttl=600
+    #
+    cut=/usr/bin/cut
+    getent=/usr/bin/getent
+    rpc_pipefs=/var/lib/nfs/rpc_pipefs
+    #
+    die()
+    {
         echo "Usage: $0 cache_name entry_name"
         exit 1
-}
+    }
 
-[ $# -lt 2 ] && die
-cachename="$1"
-cache_path=${rpc_pipefs}/cache/${cachename}/channel
+    [ $# -lt 2 ] && die
+    cachename="$1"
+    cache_path=${rpc_pipefs}/cache/${cachename}/channel
 
-case "${cachename}" in
+    case "${cachename}" in
         dns_resolve)
             name="$2"
             result="$(${getent} hosts ${name} | ${cut} -f1 -d\ )"
@@ -131,6 +137,5 @@ case "${cachename}" in
         *)
             die
             ;;
-esac
-echo "${result} ${name} ${ttl}" >${cache_path}
-
+    esac
+    echo "${result} ${name} ${ttl}" >${cache_path}