doc: extindex: note the use of `newsgroup' entries

We'll also be using this for -cindex for associating inboxes
to coderepos.

diff --git a/Documentation/public-inbox-extindex.pod b/Documentation/public-inbox-extindex.pod
index 66e04060ab94ad3d0dc4b903bd9a502dca014dc2..a5f699c515710c97b508b1da27928e8ee5fc9f8c 100644 (file)
--- a/Documentation/public-inbox-extindex.pod
+++ b/Documentation/public-inbox-extindex.pod
@@ -93,6 +93,12 @@ Putting an C<extindex> entry in the config allows L<PublicInbox::WWW>.
 You can have any number of C<extentry.$NAME> sections where C<$NAME>
 is something other than C<all> to display a union of several inboxes.
 
+It is strongly recommended any public inboxes indexed by this command
+have a stable C<publicinbox.$NAME.newsgroup> entry (regardless of
+the presence of an NNTP or IMAP server).  Otherwise, public-inbox-extindex
+will use C<publicinbox.$NAME.inboxdir> as an internal key which can
+cause needless reindexing and require L<--gc> if inboxes are relocated.
+
 See L<public-inbox-config(5)> for more details.
 
 =head1 ENVIRONMENT