Document supported/unsupported by compiler go 1.18/1.19/1.20 features

[AnnaShaleva]

Our compiler is able to handle any alias, but isn't able to handle generics (hi, #2376). We need to document this behaviour (probably, there's something else that should be mentioned in this section) and keep the documentation up-to-date.

[roman-khimov]

any is likely to be treated the same way as any other alias, so we may mention some status wrt type alias support in general.

[AnnaShaleva]

We need to write a couple of lines about unsupported generics in the Language compatibility section.

[roman-khimov]

We've got Go 1.21 now, so the list can be extended. At the same time we've got #3090, #3091 and #3092, so some of these things will be fixed.