2.13.0-M3 release notes should explain -Xsource:2.13 situation

[SethTisue]

I’d like to call it a day. maybe y’all Europeans could take care of this when you wake up and after you've eaten your crumpets or sausages or whatever you eat for breakfast

https://github.com/scala/scala/releases/tag/v2.13.0-M3 should explain the situation with -Xsource:2.13 not having been enabled, inadvertently, in M1 and M2, and what language changes (dating back to the 2.12.x branch, so not just 2.13 PRs) are now enabled as a result

[adriaanm]

Updated, and documented changes in 2.13 in #470. Could someone else make a list for 2.12 some time?

[milessabin]

Crumpets are for tea time ...