doc/develop api_lifecycle: Mention adding the arch review label

For breaking API changes, let's mention explicity the PR should
be labelled with the "Architecture Review" label, as that is
the one we base the automation on.
As otherwise it is not clear enough for developers that they
should add it, and without it (and the automation surrounding it),
it is too easy for a PR to be merged by mistake.

Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>