doc: add API helper files

These files should be a very quick reference to the Xtables APIs of
previous Linux kernel versions and Xtables-addons. Their contents have
been reformatted so as to be usable with diff -u.

doc/api/xt-a.c

@@ -0,0 +1,39 @@
+match:
+
+	/* true/false */
+	bool
+	(*match)(
+		const struct sk_buff *skb,
+		struct xt_action_param *,
+	);
+
+	/* error code */
+	int
+	(*checkentry)(
+		const struct xt_mtchk_param *,
+	);
+
+	void
+	(*destroy)(
+		const struct xt_mtdtor_param *,
+	);
+
+target:
+
+	/* verdict */
+	unsigned int
+	(*target)(
+		struct sk_buff **pskb,
+		const struct xt_action_param *,
+	);
+
+	/* error code */
+	int
+	(*checkentry)(
+		const struct xt_tgchk_param *,
+	);
+
+	void
+	(*destroy)(
+		const struct xt_tgdtor_param *,
+	);