The following patch documents that you can enable search in forums:

Index: src/search.c
==================================================================
--- src/search.c
+++ src/search.c
@@ -1981,14 +1981,15 @@
 **     reindex            Rebuild the search index.  This is a no-op if
 **                        index search is disabled
 **
 **     index (on|off)     Turn the search index on or off
 **
-**     enable cdtwe       Enable various kinds of search. c=Check-ins,
-**                        d=Documents, t=Tickets, w=Wiki, e=Tech Notes.
+**     enable cdtwef      Enable various kinds of search. c=Check-ins,
+**                        d=Documents, t=Tickets, w=Wiki, e=Tech Notes,
+**                        f=Forum.
 **
-**     disable cdtwe      Disable various kinds of search
+**     disable cdtwef     Disable various kinds of search
 **
 **     tokenizer VALUE    Select a tokenizer for indexed search. VALUE
 **                        may be one of (porter, on, off, trigram, unicode61),
 **                        and "on" is equivalent to "porter". Unindexed
 **                        search never uses tokenization or stemming.