Generics need to be documented

[wayland]

Problem or new feature

Generics don't appear to be documented

Suggestions

Document them

Context

@lizmat asked me to submit this bug report. Our investigation at the time found the following:

• https://vrurg.github.io/2020/11/16/Report-On-New-Coercions
• https://vrurg.github.io/2020/11/30/Report-On-New-Coercions-Part-2
• https://docs.raku.org/language/signatures#Type_captures

HTH,