Documentation/config.txt: avoid unnecessary negation

Documentation/config.txt: avoid unnecessary negation

Section names and variable names are both case-insensitive, but one
is described as "not case sensitive".  Use "case-insensitive" for
both.

Instead of saying "... have to be escaped" without telling what that
escaping achieves, state it in a more positive way, i.e. "... can be
included by escaping".

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