Add sync() docs.

[vpetrovykh]

I've tried different ways of writing up the details of how sync works. This is the format I've kinda landed on eventually:

• not too much implementation details (so no exact steps for order of operations, for example); this is not an RFC
• the details are split between properties, links, and link properties (as opposed to trying to split based on different stages of what sync is doing: insert, update, re-fetch)
• I ended up grouping all the multi things together as a separate section because the basics are the same as for single links and props, but there's enough of a nuance that worth highlighting because of how multi fields can be modified instead of only ever being overwritten with new values.

I also ended up not using code snippets because that seemed to me like a lot of noise that was getting in the way of keeping explanations simple. Plus we have plenty of code examples elsewhere (like primers) that we could link to instead.

[github-actions]

Docs preview deploy

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https://edgedb-docs-plhpvdbul-edgedb.vercel.app

(Last updated: Sep 8, 2025, 16:46:20 UTC)

[aljazerzen]

As someone who did not know exactly how sync worked prior to reading this, I can say it explains the method well.

The first 2 or 3 sections are very understandable and the last 3 are details to the degree that I'd skip them until I actually needed them.