doc: clarify api design guidelines for optional API

There has been discussion about whether invocations of optional API
that has been disabled should:

1. result in a link-time error because the API implementation is not
provided; or

2. result in a run-time error by having a trivial implementation that
returns -ENOTSUP.

Clarify that the intent of the guideline is that a link-time error
should be produced.  The rationale is that it is too easy to disable
the option at build time, and not discover it because the application
fails to check for a `-ENOTSUP` return.

Signed-off-by: Peter Bigot <peter.bigot@nordicsemi.no>