doc: change configuration variables format

This change configuration variables that where in italic style
to monospace font according to the guideline. It was obtained with

grep '[[:alpha:]]*\.[[:alpha:]]*::$' config.txt | \
sed -e 's/::$//' -e 's/\./\\\\./' | \
xargs -iP perl -pi -e "s/\'P\'/\`P\`/g" ./*.txt

Signed-off-by: Tom Russello <tom.russello@grenoble-inp.org>
Signed-off-by: Erwan Mathoniere <erwan.mathoniere@grenoble-inp.org>
Signed-off-by: Samuel Groot <samuel.groot@grenoble-inp.org>
Signed-off-by: Matthieu Moy <matthieu.moy@grenoble-inp.fr>
Reviewed-by: Matthieu Moy <Matthieu.Moy@imag.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>