Reindex: Allow to specify index name for reindex operation

Check the indices passed with --index option and issue error message,
when invalid index names were passed, e.g.:

  $ reindex --site-path gerrit --index foo --index bar --index baz
  fatal: invalid index name(s): [bar, baz, foo] available indices are:
  [accounts, changes]

Change-Id: Ifb60e632bd1ff3cbb364e389f045e9a0de5a23c8
Signed-off-by: Edwin Kempin <ekempin@google.com>
diff --git a/Documentation/pgm-reindex.txt b/Documentation/pgm-reindex.txt
index b58527e..f081843 100644
--- a/Documentation/pgm-reindex.txt
+++ b/Documentation/pgm-reindex.txt
@@ -27,6 +27,10 @@
 --list::
 	List available index names.
 
+--index::
+	Reindex only index with given name. This option can be supplied
+	more than once to reindex multiple indices.
+
 == CONTEXT
 The secondary index must be enabled. See
 link:config-gerrit.html#index.type[index.type].