Document when format of `*.hi` files change

Motivation

GHC does not promise that the format of *.hi files will be stable, and that is fine. However, Stack does try to make use of *.hi files, through the package hi-file-parser. If a line could be included in GHC's User Guide documentation when the format of those files change, that would help alert the Stack project to the change.

I think the most recent example is that the format has changed between GHC 9.6.3 and GHC 9.8.1 (which is at RC 1 stage).

Proposal

That GHC's User Guide documentation include a line to alert users of GHC when the format of its *.hi files has changed. (If the documentation could go further and give a pointer to how the format has changed, that would be icing on the cake.)