doc: change configuration variables format

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 <redacted>
Signed-off-by: Erwan Mathoniere <redacted>
Signed-off-by: Samuel Groot <redacted>
Signed-off-by: Matthieu Moy <redacted>
Reviewed-by: Matthieu Moy <redacted>
Signed-off-by: Junio C Hamano <redacted>