Add a note about `flush: 'sync'`, with links from elsewhere

[skirtles-code]

#1798 proposed adding information about flush: 'sync' to the main guide.

In my opinion, flush: 'sync' is an escape hatch that should be rarely used and is not something we want to encourage. Such features are typically only mentioned in the API reference, rather than the guide. I think mentioning it in an advanced guide would be fine (e.g. a detailed guide to the scheduler queue), but currently there isn't a suitable advanced guide where this topic can be covered.

The existence of flush: 'sync' is already mentioned in the API reference, but as far as I'm aware there isn't currently any explanation about what it does.

In this PR I've added a paragraph to watchEffect(), giving a brief explanation of flush: 'sync'. I have intentionally not given a lot of information or an example, for the same reason I don't think it should be in the guide.

I've also included links to watchEffect() in the entries for watch, $watch() and watch(), under the flush option. The alternative was to duplicate the explanation 4 times, which I didn't want to do.

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