doc: path.join() arguments must be strings

In v0.8, non-string arguments were ignored. v0.10 throws an exception.

doc/api/path.markdown

@@ -25,7 +25,9 @@ Example:
 ## path.join([path1], [path2], [...])
 
 Join all arguments together and normalize the resulting path.
-Non-string arguments are ignored.
+
+Arguments must be strings.  In v0.8, non-string arguments were
+silently ignored.  In v0.10 and up, an exception is thrown.
 
 Example: