Documentation: clarify pager.<cmd> configuration

It was not obvious from the text that pager.<cmd> is a boolean
setting.

While we're changing the description, make some other
improvements: lest we forget and fret, clarify that -p and
pager.<cmd> do not kick in when stdout is not a tty; point to
related core.pager and GIT_PAGER settings; use renamed --paginate
option.

Signed-off-by: Jonathan Nieder <jrnieder@uchicago.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config.txt

@@ -979,9 +979,11 @@ pack.packSizeLimit::
 	linkgit:git-repack[1].
 
 pager.<cmd>::
-	Allows to set your own pager preferences for each command, overriding
-	the default. If `\--pager` or `\--no-pager` is specified on the command
-	line, it takes precedence over this option.
+	Allows turning on or off pagination of the output of a
+	particular git subcommand when writing to a tty.  If
+	`\--paginate` or `\--no-pager` is specified on the command line,
+	it takes precedence over this option.  To disable pagination for
+	all commands, set `core.pager` or 'GIT_PAGER' to "`cat`".
 
 pull.octopus::
 	The default merge strategy to use when pulling multiple branches