docs: improve return type of getFirstDay()

[st3iny]

Nice to have after nextcloud/server#46592

[st3iny]

 lib/date.ts:21:2 - error TS2322: Type 'number' is not assignable to type '0 | 2 | 1 | 3 | 4 | 5 | 6'.

21  return window.firstDay

I guess I have to adjust our typings library as well 🙃

Or would you accept a as any or as (0 | 1 | 2 | 3 | 4 | 5 | 6) workaround?

[susnux]

I guess I have to adjust our typings library as well 🙃

Please do so :)

Or would you accept a as any or as (0 | 1 | 2 | 3 | 4 | 5 | 6) workaround?

In addition to the typings change, yes please use as (0 | 1 | 2 | 3 | 4 | 5 | 6) :)

[susnux]

You can also just define a type alias like
type WeekDay = 0 | 1 | ...