environ: explain Boolean environment variables

environ: explain Boolean environment variables

Many environment variables use the git_env_bool() API to parse their
values, and allow the usual "true/yes/on are true, false/no/off are
false. In addition non-zero numbers are true and zero is false.  An
empty string is also false." set of values.

Mark them as such, and consistently say "true" or "false", instead
of random mixes of '1', '0', 'yes', 'true', etc. in their
description.

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