feat: onpolyfill note default

[guybedford]

This updates the onpolyfill hook to apply when first executing polyfilled code instead of just on init, so when there is a real native code path that failed that the polyfill is kicking in to polyfill.

Then, the default hook contains a console.info message noting that the native error which will be present can be ignored.

Here's how that looks in Firefox:

Hopefully this will help avoid any production confusions.

[guybedford]

I've updated the note to try and be clearer: