gitignore.txt: elaborate shell glob syntax

gitignore.txt: elaborate shell glob syntax

`fnmatch(3)` is a great mention if the intended audience is
programmers. For normal users it's probably better to spell out what
a shell glob is.

This paragraph is updated to roughly tell (or remind) what the main
wildcards are supposed to do. All the details are still hidden away
behind the `fnmatch(3)` wall because bringing the whole specification
here may be too much.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>