Add documentation

Author: cherylEnkidu

firebase-firestore/src/androidTest/java/com/google/firebase/firestore/IndexingTest.java

@@ -118,6 +118,7 @@ public void testAutoIndexCreationSetSuccessfully() {
             .build();
     db.setFirestoreSettings(settings);
 
+    // Based on current heuristic, collection document counts (3) > 2 * resultSize (1).
     CollectionReference collection =
         testCollectionWithDocs(
             map(
@@ -136,13 +137,17 @@ public void testAutoIndexCreationSetSuccessfully() {
 
     results = waitFor(collection.whereEqualTo("match", true).get());
     assertEquals(1, results.size());
+
+    assertDoesNotThrow(() -> db.getPersistentCacheIndexManager().deleteAllIndexes());
+    assertEquals(1, results.size());
   }
 
   @Test
   public void testAutoIndexCreationSetSuccessfullyUsingDefault() {
     // Use persistent disk cache (default)
     FirebaseFirestore db = testFirestore();
 
+    // Based on current heuristic, collection document counts (3) > 2 * resultSize (1).
     CollectionReference collection =
         testCollectionWithDocs(
             map(
@@ -161,6 +166,9 @@ public void testAutoIndexCreationSetSuccessfullyUsingDefault() {
 
     results = waitFor(collection.whereEqualTo("match", true).get());
     assertEquals(1, results.size());
+
+    assertDoesNotThrow(() -> db.getPersistentCacheIndexManager().deleteAllIndexes());
+    assertEquals(1, results.size());
   }
 
   @Test

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

@@ -52,6 +52,10 @@ public void disableIndexAutoCreation() {
     client.setIndexAutoCreationEnabled(false);
   }
 
+  /**
+   * Removes all persistent cache indexes. Please note this function will also deletes indexes
+   * generated by {@link FirebaseFirestore#setIndexConfiguration(String)}.
+   */
   public void deleteAllIndexes() {
     client.deleteAllFieldIndexes();
   }