Document last index update timestamp storage

Feature: Issue 13901
Change-Id: I70db23365130884406cf7fc6acdcaca208a26652

src/main/resources/Documentation/about.md

diff --git a/src/main/resources/Documentation/about.md b/src/main/resources/Documentation/about.md
index fd0b56e..83d3862 100644
--- a/src/main/resources/Documentation/about.md
+++ b/src/main/resources/Documentation/about.md
@@ -103,3 +103,17 @@
 
 For further information and supported options, refer to [config](config.md)
 documentation.
+
+### Last index update timestamp storage
+
+The plugin keeps track of the timestamp when it lastly updated an index.
+When the [`autoReindex.enabled` option](config.md) is set to `true`, the timestamp
+is used to determine which changes to reindex when a node is temporarily out of
+sync with the primary, for example, after a node being offline for a long time.
+
+The HA plugin keeps the last update timestamp for each index in the following files:
+* `<gerrit_home>/data/high-availability/group`
+* `<gerrit_home>/data/high-availability/account`
+* `<gerrit_home>/data/high-availability/change`
+
+The timestamp is stored in this format `yyyy-mm-ddTHH:MM:SS.ss`, i.e.: `2020-12-18T12:17:53.25`.