Improve polyfilling doc

[EoinShaughnessy]

What

Revise polyfilling doc to:

• explain how it can help users
• make it style guide-compliant

Why

This doc is for contributors. Our tech docs workshop produced lots of good feedback about how we could enhance it. If its contents are still helpful, then it seems worthwhile to improve the doc.

Who needs to know about this

Delivery Manager, Developers, Senior Product Manager, Technical Writer

Done when

• [ ] We confirm a user need exists?
• [ ] Technical Writer revises doc
• [ ] Developers review doc
• [ ] Doc gets 2i'd
• [ ] Doc published

[EoinShaughnessy]

@hannalaakso Should this issue go into the current sprint or the backlog?

[36degrees]

Superseded by #3722