lib: reword comment about XFOLDER: prefix

I believe the current one is misleading, because in my experiments
Xapian did not add : when prefix and term were both upper case. Indeed,
it's hard to see how it could, because prefixes are added at a layer
above Xapian in our code. See _notmuch_message_add_term for an example.

Also try to explain why this is a good idea.  As far as I can ascertain,
this is more of an issue for a system trying to work with an unknown set
of prefixes. Since notmuch has a fixed set of prefixes, and we can
hopefully be trusted not to add XGOLD and XGOLDEN as prefixes, it is
harder for problems to arise.

lib/database.cc

@@ -260,10 +260,10 @@ static prefix_t BOOLEAN_PREFIX_EXTERNAL[] = {
     { "id",			"Q" },
     { "path",			"P" },
     /*
-     * Without the ":", since this is a multi-letter prefix, Xapian
-     * will add a colon itself if the first letter of the path is
-     * upper-case ASCII. Including the ":" forces there to always be a
-     * colon, which keeps our own logic simpler.
+     * Unconditionally add ':' to reduce potential ambiguity with
+     * overlapping prefixes and/or terms that start with capital
+     * letters. See Xapian document termprefixes.html for related
+     * discussion.
      */
     { "folder",			"XFOLDER:" },
 };