documentation: update CodingStyle on local variables naming in macros

Describe proper naming convention for local variables in macros
resembling functions.
Signed-off-by: NBartosz Golaszewski <bgolaszewski@baylibre.com>
Cc: Guenter Roeck <linux@roeck-us.net>
Cc: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

Documentation/CodingStyle

@@ -659,6 +659,19 @@ macros using parameters.
 #define CONSTANT 0x4000
 #define CONSTEXP (CONSTANT | 3)
 
+5) namespace collisions when defining local variables in macros resembling
+functions:
+
+#define FOO(x)				\
+({					\
+	typeof(x) ret;			\
+	ret = calc_ret(x);		\
+	(ret);				\
+)}
+
+ret is a common name for a local variable - __foo_ret is less likely
+to collide with an existing variable.
+
 The cpp manual deals with macros exhaustively. The gcc internals manual also
 covers RTL which is used frequently with assembly language in the kernel.