log: update the comment for log_msg_ret()

Add some advice on string size here.

Signed-off-by: Simon Glass <sjg@chromium.org>

include/log.h

@@ -322,7 +322,10 @@ void __assert_fail(const char *assertion, const char *file, unsigned int line,
  *
  * or:
  *
- *	return log_msg_ret("fred failed", fred_call());
+ *	return log_msg_ret("get", fred_call());
+ *
+ * It is recommended to use <= 3 characters for the name since this will only
+ * use 4 bytes in rodata
  */
 #define log_ret(_ret) ({ \
 	int __ret = (_ret); \