These functions have existed for several years now, with stable
implementation and interface. They are exposed via JS interface and used
by gltfpack, and thus are unlikely to change dramatically.

A recent change added meshopt_EncodeExpClamped, which also had a
possible alternative of adding more fine-grained exponent clamping
control to meshopt_encodeFilterExp. Instead of marking the entire
function as experimental, let's mark just the enum entry as
experimental. If more control is necessary in the future, we can remove
this enum entry and add a meshopt_encodeFilterExpClamped instead.

Fix duplicate documentation for generateShadowIndexBuffer after a failed
rewording attempt.