MINOR: counters: document that tg shared counters are tied to shm-stats-file mapping

Let's explicitly mention that fe_counters_shared_tg and
be_counters_shared_tg structs are embedded in shm_stats_file_object
struct so any change in those structs will result in shm stats file
incompatibility between processes, thus extra precaution must be
taken when making changes to them.

Note that the provisionning made in shm_stats_file_object struct could
be used to add members to {fe,be}_counters_shared_tg without changing
shm_stats_file_object struct size if needed in order to preserve
shm stats file version.

include/haproxy/counters-t.h

@@ -33,6 +33,11 @@
 	struct {                                                                     \
 		uint16_t flags;                         /* COUNTERS_SHARED_F flags */\
 	};
+
+/* /!\ any change performed here will impact shm-stats-file mapping because the
+ * struct is embedded in shm_stats_file_object struct, so proceed with caution
+ * and change shm stats file version if needed
+ */
 #define COUNTERS_SHARED_TG                                                           \
 	struct {                                                                     \
 		unsigned long last_state_change;        /* last time, when the state was changed */\
@@ -60,7 +65,11 @@ struct counters_shared {
 	} *tg[MAX_TGROUPS];
 };
 
-/* counters used by listeners and frontends */
+/*
+ * /!\ any change performed here will impact shm-stats-file mapping because the
+ * struct is embedded in shm_stats_file_object struct, so proceed with caution
+ * and change shm stats file version if needed
+ */
 struct fe_counters_shared_tg {
 	COUNTERS_SHARED_TG;
 
@@ -91,6 +100,7 @@ struct fe_counters_shared {
 	struct fe_counters_shared_tg *tg[MAX_TGROUPS];
 };
 
+/* counters used by listeners and frontends */
 struct fe_counters {
 	struct fe_counters_shared shared;       /* shared counters */
 	unsigned int conn_max;                  /* max # of active sessions */
@@ -108,6 +118,10 @@ struct fe_counters {
 	} p;                                    /* protocol-specific stats */
 };
 
+/* /!\ any change performed here will impact shm-stats-file mapping because the
+ * struct is embedded in shm_stats_file_object struct, so proceed with caution
+ * and change shm stats file version if needed
+ */
 struct be_counters_shared_tg {
 	COUNTERS_SHARED_TG;