doc: add markup for characters

This rule was already implicitely applied in the converted man pages,
so let's state it loudly.

Signed-off-by: Jean-Noël Avila <[email protected]>

Author: jnavila

Documentation/CodingGuidelines

@@ -861,6 +861,9 @@ Markup:
    _<git-dir>_
    _<key-id>_
 
+Characters are also surrounded by underscores:
+   _LF_, _CR_, _CR_/_LF_, _NUL_, _EOF_
+
  Git's Asciidoc processor has been tailored to treat backticked text
  as complex synopsis. When literal and placeholders are mixed, you can
  use the backtick notation which will take care of correctly typesetting