builtin: add documentation for `any` and `comparable` to pseudo-package `builtin`

[fzipp]

The new predeclared identifiers any and comparable in Go 1.18 should have documentation entries in the builtin pseudo-package: https://pkg.go.dev/builtin@master

[gopherbot]

Change https://golang.org/cl/368954 mentions this issue: builtin: document "any" and "comparable"

[ianlancetaylor]

Thanks for pointing that out.