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>

Documentation/gitignore.txt

@@ -102,12 +102,11 @@ PATTERN FORMAT
    (relative to the toplevel of the work tree if not from a
    `.gitignore` file).
- - Otherwise, Git treats the pattern as a shell glob suitable
-   for consumption by fnmatch(3) with the FNM_PATHNAME flag:
-   wildcards in the pattern will not match a / in the pathname.
-   For example, "Documentation/{asterisk}.html" matches
-   "Documentation/git.html" but not "Documentation/ppc/ppc.html"
-   or "tools/perf/Documentation/perf.html".
+ - Otherwise, Git treats the pattern as a shell glob: "`*`" matches
+   anything except "`/`", "`?`" matches any one character except "`/`"
+   and "`[]`" matches one character in a selected range. See
+   fnmatch(3) and the FNM_PATHNAME flag for a more detailed
+   description.
  - A leading slash matches the beginning of the pathname.
    For example, "/{asterisk}.c" matches "cat-file.c" but not