doc: update coding style

To add forward-declaration of statics rule.  This rule is already
enforced, but for some reason the document in iwd did not have this rule
in it (it is in other projects, like oFono)

doc/coding-style.txt

@@ -310,6 +310,14 @@ Example:
 0x1 << y	// Wrong
 
 
+M17: Avoid forward-declaration of static functions
+==================================================
+
+Functions that are static should not be forward-declared.  The only exception
+to this rule is if a circular dependency condition exists, and the forward
+declaration cannot be avoided.
+
+
 O1: Shorten the name
 ====================
 Better to use abbreviation, rather than full name, to name a variable,