help: clarify the choice of pager

This follows the change made in d83e51654c8a to use environment variables
between system and user configuration.

mercurial/help/pager.txt

@@ -6,9 +6,10 To set the pager that should be used, se
   [pager]
   pager = less -FRX
 
-If no pager is set, Mercurial uses the environment variable
-$PAGER. If neither pager.pager, nor $PAGER is set, a default pager
-will be used, typically `less` on Unix and `more` on Windows.
+If no pager is set in the user or repository configuration, Mercurial uses the
+environment variable $PAGER. If $PAGER is not set, pager.pager from the default
+or system configuration is used. If none of these are set, a default pager will
+be used, typically `less` on Unix and `more` on Windows.
 
 .. container:: windows