docs/misra/rules.rst: allow string literals with memcmp

Rule 21.16 is about the types of arguments allowed for memcmp. Add
string literals to the allow-list under specific conditions.

Suggested-by: Jan Beulich <jbeulich@suse.com>
Signed-off-by: Stefano Stabellini <stefano.stabellini@amd.com>
Acked-by: Jan Beulich <jbeulich@suse.com>

docs/misra/rules.rst

@@ -813,7 +813,9 @@ maintainers if you want to suggest a change.
        shall point to either a pointer type, an essentially signed type,
        an essentially unsigned type, an essentially Boolean type or an
        essentially enum type
-     - void* arguments are allowed
+     - void* arguments are allowed. string literal arguments are allowed
+       when the last argument passed for the comparison is less or equal
+       to the size of the string.
 
    * - `Rule 21.17 <https://gitlab.com/MISRA/MISRA-C/MISRA-C-2012/Example-Suite/-/blob/master/R_21_17.c>`_
      - Mandatory