Explain what not having a node param means.

doc/extending/cop.txt

@@ -274,7 +274,13 @@ needs to put variables into. But this will be clear with the examples.
 .. [#2v] There are actually two versions of this method one with a
          `c_compiler` parameter and one without. The calling code will
          try the version with c_compiler and try the version without
-         if it does not work.
+         if it does not work.  Defining both versions is pointless
+         since the one without `c_compiler` will never get called.
+
+         Note that these methods are not specific to a single apply
+         node so they may get called more than once on the same object
+         with different values for c_compiler.
+
 
 Defining the methods
 ====================