Doc: add handling of the preprocessor's "defined" op to coding style

Change-Id: I3d6e732ad45e3fcceb8d899c28ac4c4a02bb9e23
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@nokia.com>
Reviewed-by: hjk <qthjk@ovi.com>

doc/api/coding-style.qdoc

@@ -866,6 +866,17 @@
               #if Foo - 0 == 0
            \endcode
 
+        \o When checking for a preprocessor define using the \c{defined}
+           operator, always include the variable name in parentheses.
+
+            \code
+              #if defined(Foo)
+
+              -NOT-
+
+              #if defined Foo
+           \endcode
+
     \endlist
 
     \section1 Class Member Names