doc: guides: design: document how to handle conditional documentation

Various API involves functions and data that are conditionally
enabled.  Zephyr practice is to exclude data (always) and function
(sometimes) declarations at build time using the enabling preprocessor
macro.  This meets functional needs, but blocks generation of the API
documentation as Doxygen will not see the declarations.

Document the need to extend Doxygen's PREDEFINED setting to generate
the documentation.

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