Document data-fam-instance skip in reduceTypeArg

Document data-fam-instance skip in reduceTypeArg

Append a bullet to the resolution-determinism section explaining
why reduceTypeArg skips fiIsData entries: the data-family R: rewrite
makes their fiRhs structurally equal to the input FamilyApp, so
naïve recursion looped forever. Also clarify the prior
UndecidableInstances bullet — the reducer DOES recurse, it relies
on structural progress in the RHS to terminate.

Co-Authored-By: Claude Opus 4.7 (1M context) <[email protected]>