gitignore.txt: suggestions how to get literal # or ! at the beginning

We support backslash escape, but we hide the details behind the phrase
"a shell glob suitable for consumption by fnmatch(3)". So it may not
be obvious how one can get literal # or ! at the beginning of pattern.
Add a few lines on how to work around the magic characters.

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

Documentation/gitignore.txt

@@ -68,11 +68,15 @@ PATTERN FORMAT
    for readability.
 
  - A line starting with # serves as a comment.
+   Put a backslash ("`\`") in front of the first hash for patterns
+   that begin with a hash.
 
- - An optional prefix '!' which negates the pattern; any
+ - An optional prefix "`!`" which negates the pattern; any
    matching file excluded by a previous pattern will become
    included again.  If a negated pattern matches, this will
    override lower precedence patterns sources.
+   Put a backslash ("`\`") in front of the first "`!`" for patterns
+   that begin with a literal "`!`", for example, "`\!important!.txt`".
 
  - If the pattern ends with a slash, it is removed for the
    purpose of the following description, but it would only find