Rename flag and add docs

Signed-off-by: George Robinson <george.robinson@grafana.com>

cmd/alertmanager/main.go

@@ -147,7 +147,7 @@ func run() int {
 		retention           = kingpin.Flag("data.retention", "How long to keep data for.").Default("120h").Duration()
 		maintenanceInterval = kingpin.Flag("data.maintenance-interval", "Interval between garbage collection and snapshotting to disk of the silences and the notification logs.").Default("15m").Duration()
 		maxSilences         = kingpin.Flag("silences.max-silences", "Maximum number of active and pending silences. If negative or zero, no limit is set.").Default("0").Int()
-		maxSilenceSize      = kingpin.Flag("silences.max-size", "Maximum per silence size (in bytes). If negative or zero, no limit is set.").Default("0").Int()
+		maxPerSilenceSize   = kingpin.Flag("silences.max-per-silence-size", "Maximum per silence size (in bytes). If negative or zero, no limit is set.").Default("0").Int()
 		alertGCInterval     = kingpin.Flag("alerts.gc-interval", "Interval between alert GC.").Default("30m").Duration()
 
 		webConfig      = webflag.AddFlags(kingpin.CommandLine, ":9093")
@@ -263,7 +263,7 @@ func run() int {
 		Retention:    *retention,
 		Limits: silence.Limits{
 			MaxSilences:       *maxSilences,
-			MaxPerSilenceSize: *maxSilenceSize,
+			MaxPerSilenceSize: *maxPerSilenceSize,
 		},
 		Logger:  log.With(logger, "component", "silences"),
 		Metrics: prometheus.DefaultRegisterer,

docs/configuration.md

@@ -22,6 +22,14 @@ is not well-formed, the changes will not be applied and an error is logged.
 A configuration reload is triggered by sending a `SIGHUP` to the process or
 sending an HTTP POST request to the `/-/reload` endpoint.
 
+## Limits
+
+Alertmanager supports a number of configurable limits via command-line flags.
+
+To limit the maximum number of active and pending silences, use the `--silences.max-silences` flag.
+You can limit the maximum size of individual silences with `--silences.max-per-silence-size`,
+where the unit is in bytes.
+
 ## Configuration file introduction
 
 To specify which configuration file to load, use the `--config.file` flag.