doc: update function references to use :c:func:

Following the change to use the C domain for parsing, update all
existing :cpp:func: references to :c:func:. Remove the parentheses as
well, if used, because they are not needed, this is already known to be
a function, and how it is displayed in the documentation later is a
semantic decision done by the output builder.

Signed-off-by: Fabio Utzig <fabio.utzig@nordicsemi.no>