Init: Add documentation for --reindex-threads

Release-Notes: skip
Change-Id: Ie7fe6461752ff4088ade69ed2ecbbc8183fbce11
diff --git a/Documentation/pgm-init.txt b/Documentation/pgm-init.txt
index 9f592486..b28a617 100644
--- a/Documentation/pgm-init.txt
+++ b/Documentation/pgm-init.txt
@@ -20,6 +20,7 @@
   [--dev]
   [--skip-all-downloads]
   [--skip-download=<LIBRARY_NAME>]
+  [--reindex-threads=<N>]
 --
 
 == DESCRIPTION
@@ -102,6 +103,9 @@
 --show-cache-stats::
 	Show cache statistics at the end of program.
 
+--reindex-threads::
+	Number of threads to use for reindex after init. Defaults to 1.
+
 == CONTEXT
 This command can only be run on a server which has direct local access to the
 managed Git repositories.