doc: Add net.connman.iwd.IPv{4,6}Configuration API doc

doc/ip-configuration-api.txt

@@ -0,0 +1,47 @@
+IP Configuration hierarchy
+===============================
+
+Service		net.connman.iwd
+Interface	net.connman.iwd.IPv4Configuration [Experimental]
+Interface	net.connman.iwd.IPv6Configuration [Experimental]
+Object path	/net/connman/iwd/{phy0,phy1,...}/{1,2,...}
+Object path	/net/connman/iwd/{phy0,phy1,...}/p2p_peers/{aa_bb_cc_dd_ee_ff}
+
+The interfaces net.connman.iwd.IPv4Configuration and
+net.connman.iwd.IPv6Configuration currently have the same sets of methods,
+signals and properties.  In station mode, when network configuration is
+enabled there may be one or both interfaces present on a device object in
+connected state depending on if IPv4 and IPv6 addresses have both been
+configured.  In P2P mode only net.connman.iwd.IPv4Configuration is used.
+
+Properties	string Method [readonly]
+
+			Indicates whether the local address was set
+			statically (value "static") or obtained automatically
+			such as through DHCP (value "auto").  Even when the
+			address was obtained from the remote end some
+			configuration bits, such as DNS addresses, may have
+			been overridden locally.
+
+		string Address [readonly]
+
+			Holds the local IP address.
+
+		byte PrefixLength [readonly]
+
+			Holds the prefix-length of the local subnet.  For
+			IPv4 this maps to the netmask.
+
+		string Gateway [readonly, optional]
+
+			Holds the gateway address for the subnet if one
+			exists.
+
+		array(string) DomainNameServers [readonly, optional]
+
+			Holds the list of domain name server addresses
+			configured if any.
+
+		array(string) DomainNames [readonly, optional]
+
+			Holds the network's local domain names if any exist.