Missing explanation of the Callback Flush Timing `'sync'` option

[DrJume]

https://vuejs.org/guide/essentials/watchers.html#callback-flush-timing

[skirtles-code]

I suspect it's intentionally missing from that section, as sync is an escape-hatch for edge cases and not necessarily something that needs to be covered in the main guide. However, it doesn't currently seem to be explained anywhere else, and I'd definitely agree that it needs to be explained somewhere.

[skirtles-code]

A brief explanation of flush: 'sync' was added in #1895. The explanation is in the API docs, rather than the guide.