doc: clearer rule about formatting literals

Make the guideline text that we want for our documentation clearer.

Signed-off-by: Tom Russello <tom.russello@grenoble-inp.org>
Signed-off-by: Erwan Mathoniere <erwan.mathoniere@grenoble-inp.org>
Signed-off-by: Samuel Groot <samuel.groot@grenoble-inp.org>
Signed-off-by: Matthieu Moy <matthieu.moy@grenoble-inp.fr>
Reviewed-by: Matthieu Moy <Matthieu.Moy@imag.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/CodingGuidelines

@@ -526,12 +526,19 @@ Writing Documentation:
  modifying paragraphs or option/command explanations that contain options
  or commands:
 
- Literal examples (e.g. use of command-line options, command names, and
- configuration variables) are typeset in monospace, and if you can use
- `backticks around word phrases`, do so.
+ Literal examples (e.g. use of command-line options, command names,
+ configuration and environment variables) must be typeset in monospace (i.e.
+ wrapped with backticks):
    `--pretty=oneline`
    `git rev-list`
    `remote.pushDefault`
+   `GIT_DIR`
+
+ An environment variable must be prefixed with "$" only when referring to its
+ value and not when referring to the variable itself, in this case there is
+ nothing to add except the backticks:
+   `GIT_DIR` is specified
+   `$GIT_DIR/hooks/pre-receive`
 
  Word phrases enclosed in `backtick characters` are rendered literally
  and will not be further expanded. The use of `backticks` to achieve the