Document that the change search API might return stale results

Also mention that updates to the project config do not result in change
re-indexing.

Release-Notes: skip
Google-Bug-Id: b/265653434
Change-Id: I35356db72c33102a006e9a8ec1e1f7ac29c5e7e2

Documentation/user-search.txt

diff --git a/Documentation/user-search.txt b/Documentation/user-search.txt
index 565c491..e12c27c 100644
--- a/Documentation/user-search.txt
+++ b/Documentation/user-search.txt
@@ -43,6 +43,17 @@
 For more predictable results, use explicit search operators as described
 in the following section.
 
+[IMPORTANT]
+--
+The change search API is backed by a secondary index and might sometimes return
+stale results if the re-indexing operation failed for a change update.
+
+Please also note that changes are not re-indexed if the project configuration
+is updated with newly added or modified
+link:config-submit-requirements.html[submit requirements].
+--
+
+
 [[search-operators]]
 == Search Operators