Docs: Document how strings work for defaultChecked

[tordans]

This adds anchor links to the field type definitions to clarify what "combo field types" are.

It also tries to document how the strings work with checkbox field types.

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@Zverik Does this sound right to you? I looked into this after reading your comment at Zverik/every_door#617 (comment)

[Zverik]

Thanks Tobias! I think a clarification is needed still: looks like for checkboxes (and defaultCheck), options should follow the fixed [undefined, yes, no] order: first is always undefined, then a value for "checked", and a value for "unchecked" (in case of a check field; defaultCheck unsets value for unchecked).