Update add/remove observer methods documentation to clarify threading

Test: n/a
Bug: 153948821
Relnote: Update add/remove observer methods documentation to clarify threading

Change-Id: Ifd1d988bf88ca54415aca08e82ed9e45a654a02c
diff --git a/room/runtime/src/main/java/androidx/room/InvalidationTracker.java b/room/runtime/src/main/java/androidx/room/InvalidationTracker.java
index 980aba5..45d819e 100644
--- a/room/runtime/src/main/java/androidx/room/InvalidationTracker.java
+++ b/room/runtime/src/main/java/androidx/room/InvalidationTracker.java
@@ -249,6 +249,9 @@
      * <p>
      * If one of the tables in the Observer does not exist in the database, this method throws an
      * {@link IllegalArgumentException}.
+     * <p>
+     * This method should be called on a background/worker thread as it performs database
+     * operations.
      *
      * @param observer The observer which listens the database for changes.
      */
@@ -322,6 +325,9 @@
 
     /**
      * Removes the observer from the observers list.
+     * <p>
+     * This method should be called on a background/worker thread as it performs database
+     * operations.
      *
      * @param observer The observer to remove.
      */