Add descriptions of built-in Readonly types like ReadonlyMap, ReadonlySet, etc.

[lindner]

In #798 descriptions of various utility types were added. What's missing are some of the very basic, very useful types ReadonlyMap, ReadonlySet etc. Would be good to add those so people don't recreate them on their own.

[karlhorky]

Add ReadonlyArray to this list too!

For now the closest thing to documentation for ReadonlyArray is this: https://www.typescriptlang.org/docs/handbook/interfaces.html#readonly-properties

I've opened an issue for the rest of the common utility types: https://github.com/microsoft/TypeScript-Handbook/issues/1060