Undocumented change to the `File` type definition

[blamario]

The File type synonym used to be defined as a pair whose first component was lazy Text. With version 0.21 it became strict Text instead. I think this was a good change, except it's backward incompatible. There's no going back now, but it should be documented in the changelog.

[ocramz]

@blamario thank you for reporting, we're usually more careful with user-facing changes.