Datepicker: Add documentation for onUpdateDatepicker callback option

[patrick-vandy]

Update datepicker's documentation for changes made in jquery/jquery-ui#1912

[mgol]

The change looks good, thanks!

@fnagel are you familiar with how the docs for a specific version are generated? I wonder if updating the main branch wouldn't update the 1.12 docs. If that's the case then we need to prepare a 1.13 setup before merging this, let's keep this in mind.

[fnagel]

@mgol No, I'm not familiar, but you're right, we should keep this in mind. Did you create a checklist somewhere?

[mgol]

I was thinking about doing the API setup for 1.13 before merging this but we can't as the main URLs like https://api.jqueryui.com/addClass/ need to resolve to 1.12 as long as 1.13 is not released yet. This is good to merge then, we just need to not tag commits on main before the 1.13 release; if any 1.12 docs need updates, we can create tags on the 1-12 branch.

I'll merge the PR.

[mgol]

Landed, thanks for the docs update, @patrick-vandy!