Documentation: Remove "source" key when using meta attributes

[prtksxna]

When the source key is added, the elements don't get updated in
their lifecycle but the data gets saved correctly. Based on @pento's
Gist, removing this key fixes the problem.

https://gist.github.com/pento/19b35d621709042fc899e394a9387a54

[aduth]

Since #2854, we should assume any attribute included in attributes should define its source, except for comment attributes. Could you share an example of a problematic meta-based block, in case there's an underlying issue that needs to be resolved?

cc @youknowriad

[prtksxna]

Hey @aduth o/ Thank you for your response, and sorry for replying so late.

If you see the gist that I linked to you'll notice that it currently does not have source: "meta". If you do add it then the meta data doesn't get saved. Hitting update doesn't actually save the value, and when you refresh the editor the value is back to zero.

I am testing on Firefox 59, on Mac OS, on version 245035b.

[youknowriad]

@prtksxna You're right and that's a bug, it's probably related to the fix I pushed yesterday #3754 It should behave better in the next release. Dropping the "source" makes it work because it's not saved in post_meta anymore but it's saved as a "comment" attribute.

[aduth]

Based on #3652 (comment), considering this superseded by #3754. Thanks for the pull request and issue report.

[prtksxna]

In that case I'll close this PR. In your PR #3754 I could not find a reference to the issue though. Could you please point me to it so that I can subscribe?

Thanks @aduth and @youknowriad!

[prtksxna]

Oops! Sorry for the misleading comment @aduth, I had kept this tap open for too long and did not see your reply before drafting that comment. Thanks again.