doc: Add a man page for PPP dial-up interfaces

Signed-off-by: Maximilian Wilhelm <max@sdn.clinic>

Makefile

@@ -147,6 +147,7 @@ MANPAGES_5 = \
 	doc/interfaces-bond.5 \
 	doc/interfaces-batman.5 \
 	doc/interfaces-bridge.5 \
+	doc/interfaces-ppp.5 \
 	doc/interfaces-vrf.5 \
 	doc/interfaces-vxlan.5 \
 	doc/interfaces-wireguard.5

doc/interfaces-ppp.scd

@@ -0,0 +1,37 @@
+interfaces-ppp(5)
+
+# NAME
+
+*interfaces-ppp* - PPP extensions for the interfaces(5) file format
+
+# DESCRIPTION
+
+The Point-to-Point Protocol (PPP) usually is used for dial-up lines,
+most common for over Digital Subscriber Lines (DSL) to connect to an
+Internet Service Provider (ISP). The following options allow to set
+up PPP dial-up connections.
+
+# PPP-RELATED OPTIONS
+
+*ppp-provider* _provider_
+	Denotes the file name of the _provider_ configuration file
+	within the _/etc/ppp/peers/_ directory which should be used
+	to set up the PPP connection.
+
+*ppp-physdev* _interfaces_
+	Denotes the physical (underlay) interface which is used to
+	set up the PPP connection.
+
+# EXAMPLES
+
+A PPP connection to _local-ISP_:
+
+```
+auto ppp0
+iface ppp0
+	ppp-provider local-ISP
+```
+
+# AUTHORS
+
+Maximilian Wilhelm <max@sdn.clinic>

doc/interfaces.scd

@@ -148,6 +148,11 @@ most common options are:
 *loopback*
 	Designates the interface as a loopback device.
 
+*ppp*
+	Designates the interface as a PPP device.  Configuration
+	of PPP interfaces require the *ppp* and probably the *pppoe*
+	packages to be installed.
+
 *tunnel*
 	The interface is a tunnel.  Configuration of tunnels
 	requires the *tunnel* package to be installed.
@@ -205,6 +210,7 @@ iface eth0
 *interfaces-batman*(5)
 *interfaces-bond*(5)
 *interfaces-bridge*(5)
+*interfaces-ppp*(5)
 *interfaces-vrf*(5)
 *interfaces-vxlan*(5)
 *interfaces-wireguard*(5)