Documentation: clarify meaning of --html-path, --man-path, and --info-path

Documentation: clarify meaning of --html-path, --man-path, and --info-path

These options tell UI programs where git put its documentation, so
"Help" actions can show the documentation for *this* version of git
without regard to how MANPATH and INFOPATH are set up.  Details:

. Each variable tells where documentation is expected to be.  They do
  not indicate whether documentation was actually installed.

. The output of "git --html-path" is an absolute path and can be used
  in "file://$(git --html-path)/git-add.html" to name the HTML file
  documenting a particular command.

. --man-path names a manual page hierarchy (e.g.,
  /home/user/share/man).  Its output can be passed to "man -M" or put
  at the beginning of $MANPATH.

. --info-path names a directory with info files (e.g.,
  /home/user/share/info).  Its output is suitable as an argument to
  "info -d" or for inclusion in $INFOPATH.

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