Add AnimationFrame package link and update docs for Time.every

[rl-king]

Updated the docs for Time.every, referring/linking to the AnimationFrame package in the note. Someone might interpret this as requestAnimationFrame not being available in Elm without a port (furrary/react-fiber-demo#3 (comment)).

note: I wasn't able to test this as elm make docs didn't seem to work.

[process-bot]

Thanks for the pull request! Make sure it satisfies this checklist. My human colleagues will appreciate it!

Here is what to expect next, and if anyone wants to comment, keep these things in mind.

[process-bot]

Thanks for the pull request! Make sure it satisfies this checklist. My human colleagues will appreciate it!

Here is what to expect next, and if anyone wants to comment, keep these things in mind.