CodingStyle: Use of common sense is mandatory

Make it explicit that the use of common sense is required when applying
the rules of the style guide.  Also point out explicitly that in
multiple variable declarations, you're supposed to "violate" an earlier
rule.

Reviewed-by: Gleb Natapov <gleb@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>