doc: check-ignore: code-quote an exclamation mark

The plain quoted exclamation mark renders as italics in the
Windows pdf help manual.

Fix this with back-tick quoting and surrounding double quotes
as exemplified by the gitignore.txt guide.

While at it, fix  the surrounding double quotes for the other
special characters usages.

Signed-off-by: Philip Oakley <philipoakley@iee.email>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-check-ignore.txt

@@ -33,7 +33,7 @@ OPTIONS
 	Instead of printing the paths that are excluded, for each path
 	that matches an exclude pattern, print the exclude pattern
 	together with the path.  (Matching an exclude pattern usually
-	means the path is excluded, but if the pattern begins with '!'
+	means the path is excluded, but if the pattern begins with "`!`"
 	then it is a negated pattern and matching it means the path is
 	NOT excluded.)
 +
@@ -77,7 +77,7 @@ If `--verbose` is specified, the output is a series of lines of the form:
 <pathname> is the path of a file being queried, <pattern> is the
 matching pattern, <source> is the pattern's source file, and <linenum>
 is the line number of the pattern within that source.  If the pattern
-contained a `!` prefix or `/` suffix, it will be preserved in the
+contained a "`!`" prefix or "`/`" suffix, it will be preserved in the
 output.  <source> will be an absolute path when referring to the file
 configured by `core.excludesFile`, or relative to the repository root
 when referring to `.git/info/exclude` or a per-directory exclude file.