document serviceWorker register option

[ignatiusmb]

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[benmccann]

the current wording is slightly ambiguous as it can be read as "it is set to false so that it will be disabled" or "you can set it to false to disable"

Suggested change

	- `register` - set to `false` to disable automatic service worker registration
	- `register` - whether to enable automatic service worker registration. Default is `true`

[ignatiusmb]

I'm following a similar wording style to the prerender.enabled config, and we don't really mention the default value for the config anywhere since it's already displayed in the code block at the start, so I don't think the last part is necessary