Add "IEEE Std 1003.1, 2016 edition" to mandoc and mdoc.7 per discussion in D13334, though I didn't add the intermediate 2013 edition as it is, well, intermediate and should be superseded by 2016 one -- of course, if I'm understanding it correctly, if not, I'll add it as well.
If this is accepted, I'll try upstreaming the change.