pager: document the 'pager.enable' option stable

pager: document the 'pager.enable' option

The 'config' helps was missing help about pager enabling/disabling.

--- a/mercurial/help/config.txt	Mon May 01 18:07:23 2017 +0200
+++ b/mercurial/help/config.txt	Mon May 01 16:43:43 2017 +0200
@@ -1367,6 +1367,9 @@
 Setting used to control when to paginate and with what external tool. See
 :hg:`help pager` for details.
 
+``enable``
+  Control the pagination of command output (default: True).
+
 ``pager``
     Define the external tool used as pager.