Update docs for change in parsing behaviour of infix operators like `@` in GHC 9

GHC 9 now treats @ with spaces as an infix operator, which breaks Template Haskell code like:

let myType = ... :: Q Type
[| foo @ $myType |]

(this is intended to generate code like foo @Int). The space here was needed so that @$ wasn't being treated as a single unit, but this breaks with GHC 9. The fix is to change this to @($myType).

Update migration guide, release notes, and any other documentation to be more clear about this change in behavior. There was a small note added to the release docs in !5499 (closed), but it doesn't clearly indicate that it breaks code like the above.