Documentation: No argument of ALLOC_GROW should have side-effects

The explanatory comment before the definition of ALLOC_GROW carefully
lists arguments that will be used more than once and thus cannot have
side-effects; a lazy reader might conclude that the arguments not
listed are used only once and side effects safe.

Correct it to list all three arguments, avoiding this confusion.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

cache.h

@@ -445,7 +445,7 @@ extern int init_db(const char *template_dir, unsigned int flags);
  * at least 'nr' entries; the number of entries currently allocated
  * is 'alloc', using the standard growing factor alloc_nr() macro.
  *
- * DO NOT USE any expression with side-effect for 'x' or 'alloc'.
+ * DO NOT USE any expression with side-effect for 'x', 'nr', or 'alloc'.
  */
 #define ALLOC_GROW(x, nr, alloc) \
 	do { \