document internal here (was already documented in plugins/aggregate)

doc/ikiwiki/directive/aggregate.mdwn

@@ -19,6 +19,11 @@ more aggregated feeds. For example:
 
 	\[[!inline pages="internal(example/*)"]]
 
+Note the use of `internal()` in the [[ikiwiki/PageSpec]] to match
+aggregated pages. By default, aggregated pages are internal pages,
+which prevents them from showing up directly in the wiki, and so this
+special [[PageSpec]] is needed to match them.
+
 ## usage
 
 Here are descriptions of all the supported parameters to the `aggregate`