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>

Documentation/gitignore.txt

@@ -90,12 +90,12 @@ Patterns have the following format:
  - 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/\*.html" matches
+   For example, "Documentation/{asterisk}.html" matches
    "Documentation/git.html" but not "Documentation/ppc/ppc.html"
    or "tools/perf/Documentation/perf.html".
 
  - A leading slash matches the beginning of the pathname.
-   For example, "/*.c" matches "cat-file.c" but not
+   For example, "/{asterisk}.c" matches "cat-file.c" but not
    "mozilla-sha1/sha1.c".
 
 An example: