Documentation: clarify quoting in gitignore docs

Documentation: clarify quoting in gitignore docs

An asterisk in "Documentation/*.txt" quoted with \ to avoid bold text
is being output as \* because asciidoc does not consider it a
candidate for escaping (there is no matching * to pair it with).

So the manual looks like it is saying that one should write
"Documentation/\*.txt" in the .gitignore file.

Reported-by: Frédéric Brière <fbriere@fbriere.net>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>