docs(Collector): properly document endReason (#6016)

src/structures/MessageCollector.js

@@ -89,6 +89,11 @@ class MessageCollector extends Collector {
     return message.channel.id === this.channel.id ? message.id : null;
   }
 
+  /**
+   * The reason this collector has ended with, or null if it hasn't ended yet
+   * @type {?string}
+   * @readonly
+   */
   get endReason() {
     if (this.options.max && this.collected.size >= this.options.max) return 'limit';
     if (this.options.maxProcessed && this.received === this.options.maxProcessed) return 'processedLimit';