Config changes

- a source's interval must be greater than 0: a source is turned off with
  enabled, not with a zero interval, so JSCSourcesSource rejects an interval of
  0 or less (its validate is promoted to each source's section);
- max_message_size rejects -1: JSCServerWebSocket rejects a max_message_size
  of -1 (util.ByteSizeNoLimit), as a client could then exhaust the server's
  memory;
- every setting documents what 0 or empty means (zero/none, no limit or a
  default).

internal/config/conf_ratelimitrule.go

@@ -2,12 +2,14 @@ package config
 
 import "reflect"
 
-// JSCRateLimitRule configures one rate limit. The type is used under several
-// keys so it has no JSCK* constant and its Flatten keys are not prefixed.
+// JSCRateLimitRule configures one rate limit: PerSecond sustained, with
+// bursts of up to Burst. It is off unless Enabled and PerSecond is greater
+// than 0; a Burst below 1 means 1. The type is used under several keys so it
+// has no JSCK* constant and its Flatten keys are not prefixed.
 type JSCRateLimitRule struct {
-	Enabled   bool `cfg:"enabled"             default:"false"`
+	Enabled   bool `cfg:"enabled"    default:"false"`
 	PerSecond int  `cfg:"per_second" default:"50"`
-	Burst     int  `cfg:"burst"               default:"100"`
+	Burst     int  `cfg:"burst"      default:"100"`
 }
 
 // ---------- Section methods (see Section) ----------