docs: networking: Convert alias.txt to rst

The kernel documentation is now restructured text. Convert the IP
aliasing documentation and include it in the toplevel kernel
documentation.

 - Fix heading adornments.
 - Correctly indent code snippets.
 - Limit line length to 72 characters inline with kernel documentation
   standards.
 - Add license identifier.

Signed-off-by: Tobin C. Harding <me@tobin.cc>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/networking/alias.rst

@@ -0,0 +1,49 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+===========
+IP-Aliasing
+===========
+
+IP-aliases are an obsolete way to manage multiple IP-addresses/masks
+per interface. Newer tools such as iproute2 support multiple
+address/prefixes per interface, but aliases are still supported
+for backwards compatibility.
+
+An alias is formed by adding a colon and a string when running ifconfig.
+This string is usually numeric, but this is not a must.
+
+
+Alias creation
+==============
+
+Alias creation is done by 'magic' interface naming: eg. to create a
+200.1.1.1 alias for eth0 ...
+::
+
+  # ifconfig eth0:0 200.1.1.1  etc,etc....
+	~~ -> request alias #0 creation (if not yet exists) for eth0
+
+The corresponding route is also set up by this command.  Please note:
+The route always points to the base interface.
+
+
+Alias deletion
+==============
+
+The alias is removed by shutting the alias down::
+
+  # ifconfig eth0:0 down
+	~~~~~~~~~~ -> will delete alias
+
+
+Alias (re-)configuring
+======================
+
+Aliases are not real devices, but programs should be able to configure
+and refer to them as usual (ifconfig, route, etc).
+
+
+Relationship with main device
+=============================
+
+If the base device is shut down the added aliases will be deleted too.