Stable features warning should note the version stabilized, if available.

[Aatch]

I use several feature gates in Ramp, two of which are now stable. However, since I use the nightly compiler, I have no idea if they're stable in the stable build of the compiler without going and investigating when they were stabilized. It would be nice if it said which version they were stabilized in so I just have to check which version the stable release is.

[zackmdavis]

I think I've made some progress on this ...