doc: document the --prune option for -index

We've had it around for a while, but I forgot to document it :x

diff --git a/Documentation/public-inbox-index.pod b/Documentation/public-inbox-index.pod
index 6d2a42031cfb121b5a986d0085b73f47454d9a06..7679376c3664e76d6196d65aee15caaae55bd9a5 100644 (file)
--- a/Documentation/public-inbox-index.pod
+++ b/Documentation/public-inbox-index.pod
@@ -42,6 +42,13 @@ Xapian database.
 
 This does not touch the NNTP article number database.
 
+=item --prune
+
+Run L<git-gc(1)> to prune and expire reflogs if discontiguous history
+is detected.  This is intended to be used in mirrors after running
+L<public-inbox-edit(1)> or L<public-inbox-purge(1)> to ensure data
+is expunged from mirrors.
+
 =back
 
 =head1 FILES