Add clarification about commitWithin setting and reindex

If the commitWithin setting is low, it can cause the reindex to
run slowly.  Add a note in the documentation about this.

Change-Id: If4c394443bde89be4bfc1505d33322089e671493

Documentation/config-gerrit.txt

diff --git a/Documentation/config-gerrit.txt b/Documentation/config-gerrit.txt
index 66f9d82..be6efc6 100644
--- a/Documentation/config-gerrit.txt
+++ b/Documentation/config-gerrit.txt
@@ -1892,6 +1892,12 @@
 very costly but may be useful if offline reindexing is infeasible, or
 for development servers.
 +
+This setting also applies when running the reindex program.  If it is
+configured to commit on every write, this will cause reindex to take
+an unnecessarily long time to complete on sites that have a lot of
+changes.  It is recommended to temporarily set a higher value while
+running reindex.
+
 Defaults to 300000 ms (5 minutes).
 
 Sample index configuration: