config: rewrite core.pager documentation

The text mentions core.pager and GIT_PAGER without giving the
overall picture of precedences.  Borrow a better description from
the git-var(1) documentation.

The use of the mechanism to allow system-wide, global and
per-repository configuration files is not limited to this particular
variable.  Remove it to clarify the paragraph.

Rewrite the part that explains how the environment variable LESS is
set to Git's default value, and how to selectively customize it.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config.txt

@@ -561,22 +561,20 @@ sequence.editor::
 	When not configured the default commit message editor is used instead.
 
 core.pager::
-	The command that Git will use to paginate output.  Can
-	be overridden with the `GIT_PAGER` environment
-	variable.  Note that Git sets the `LESS` environment
-	variable to `FRSX` if it is unset when it runs the
-	pager.  One can change these settings by setting the
-	`LESS` variable to some other value.  Alternately,
-	these settings can be overridden on a project or
-	global basis by setting the `core.pager` option.
-	Setting `core.pager` has no effect on the `LESS`
-	environment variable behaviour above, so if you want
-	to override Git's default settings this way, you need
-	to be explicit.  For example, to disable the S option
-	in a backward compatible manner, set `core.pager`
-	to `less -+S`.  This will be passed to the shell by
-	Git, which will translate the final command to
-	`LESS=FRSX less -+S`.
+	Text viewer for use by Git commands (e.g., 'less').  The value
+	is meant to be interpreted by the shell.  The order of preference
+	is the `$GIT_PAGER` environment variable, then `core.pager`
+	configuration, then `$PAGER`, and then the default chosen at
+	compile time (usually 'less').
++
+When the `LESS` environment variable is unset, Git sets it to `FRSX`
+(if `LESS` environment variable is set, Git does not change it at
+all).  If you want to selectively override Git's default setting
+for `LESS`, you can set `core.pager` to e.g. `less -+S`.  This will
+be passed to the shell by Git, which will translate the final
+command to `LESS=FRSX less -+S`. The environment tells the command
+to set the `S` option to chop long lines but the command line
+resets it to the default to fold long lines.
 
 core.whitespace::
 	A comma separated list of common whitespace problems to