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>
Merge branch 'jc/pager-configuration-doc' into pu

* jc/pager-configuration-doc:
  config: rewrite core.pager documentation