Add a couple comments on breadcrumb config and early exit behaviors

lib/client.js

@@ -43,6 +43,10 @@ extend(Raven.prototype, {
     this.release = options.release || process.env.SENTRY_RELEASE || '';
     this.environment = options.environment || process.env.SENTRY_ENVIRONMENT || '';
 
+    // autoBreadcrumbs: true enables all, autoBreadcrumbs: false disables all
+    // autoBreadcrumbs: { http: true } enables a single type
+    // this procedure will ensure that this.autoBreadcrumbs is an object populated
+    // with keys -> bools reflecting actual status of all breadcrumb types
     var autoBreadcrumbDefaults = {
       console: false,
       http: false,
@@ -409,6 +413,7 @@ extend(Raven.prototype, {
   },
 
   captureBreadcrumb: function (breadcrumb) {
+    // Avoid capturing global-scoped breadcrumbs before instrumentation finishes
     if (!this.installed) return;
 
     breadcrumb = extend({