Doc: build: clarify why dependencies are needed

Author: kgaillot

INSTALL.md

@@ -25,9 +25,9 @@
 * systemd-devel (systemd support)
 * dbus-devel (systemd/upstart resource support)
 * cluster-glue-libs-devel (LHA style fencing agents)
-* libesmtp-devel (Email alerts)
-* lm_sensors-devel (SNMP alerts)
-* net-snmp-devel (SNMP alerts)
+* libesmtp-devel (crm_mon --mail-to option)
+* lm_sensors-devel (crm_mon --snmp-traps option)
+* net-snmp-devel (crm_mon --snmp-traps option)
 * asciidoc (documentation)
 * help2man (documentation)
 * publican (documentation)