docs/misra: Document ECLAIR extension to Rule 20.7

MISRA C Rule 20.7 states:
"Expressions resulting from the expansion of macro parameters shall
be enclosed in parentheses".

Document the behaviour of ECLAIR with respect to the CPP extension
that allows variable macro arguments to be named.

Signed-off-by: Nicola Vetrini <nicola.vetrini@bugseng.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>

docs/misra/rules.rst

@@ -671,7 +671,14 @@ maintainers if you want to suggest a change.
        shall be enclosed in parentheses
      - Extra parentheses are not required when macro parameters are used
        as function arguments, as macro arguments, array indices, lhs in
-       assignments or as initializers in initalizer lists.
+       assignments or as initializers in initalizer lists. In addition,
+       the use of a named variable argument in a macro that would constitute
+       a violation of the rule is allowed by ECLAIR as an extension of the
+       MISRA guideline, since it may not always be possible to parenthesize
+       such argument and the feature is non-standard::
+
+         #define M(args...) args
+         #if M(1) + 0 
 
    * - `Rule 20.9 <https://gitlab.com/MISRA/MISRA-C/MISRA-C-2012/Example-Suite/-/blob/master/R_20_09.c>`_
      - Required