[checks] Add check for \pnum Implementations should #8548

We have a surprising amount of violations of the form

\pnum
Implementations should ...

I'm pretty sure that every one of those ought to be

\pnum
\recommended
Implementations should ...

There are also some "Implementations should" inside of notes and examples, which may be a bit undesirable, but it's not obvious whether or how to fix those cases. It's also harder to detect/fix the cases where "Implementations should" appears in some longer paragraph.