AggregateSource.java: re-write the javadocs

Author: dconeybe

firebase-firestore/src/main/java/com/google/firebase/firestore/AggregateSource.java

@@ -14,13 +14,23 @@
 
 package com.google.firebase.firestore;
 
-/** Configures the behavior of {@link AggregateQuery#get}. */
+/**
+ * The sources from which an {@link AggregateQuery} can retrieve its results.
+ *
+ * @see AggregateQuery#get
+ */
 public enum AggregateSource {
   /**
-   * Reach to the Firestore backend and surface the result verbatim, that is no local documents or
-   * mutations in the SDK cache will be included in the surfaced result.
+   * Perform the aggregation on the server and download the result.
    *
-   * <p>Requires client to be online.
+   * <p>The result received from the server is presented, unaltered, without considering any local
+   * state. That is, any documents in the local cache are ignored and any documents that have been
+   * locally modified but not yet synchronized with the server are not taken into account. The
+   * result received from the server is not cached for later use: every request using this source
+   * necessarily involves a round trip to the server and back.
+   *
+   * <p>The {@link AggregateQuery} will fail if the server cannot be reached, such as if the client
+   * is offline.
    */
   SERVER,
 }