document how to make a global comments feed

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+You've enabled the [[plugins/comments]] plugin, so a set of pages on your
+blog can have comments added to them. Pages with comments even have special
+feeds that can be used to subscribe to those comments. But you'd like to
+add a feed that contains all the comments posted to any page. Here's how:
+
+       \[[!inline pages="internal(*/comment_*)" template=comments_display]]
+
+The special [[ikiwiki/PageSpec]] matches all comments. The
+[[template|wikitemplates]] causes the comments to be displayed formatted
+nicely.