Add documentation for segment email send button being unavailable if publish up date has passed

[RCheesley]

See this PR: mautic/mautic#11351

When the publish-up date for a segment email is in the past, the email should be getting sent, so the send button will be greyed out to prevent the duplication of email sends.

⚠️ As the PR is on the 5.x branch, this will only apply to the Mautic 5 release, so we will need to make this change on the 5.0 branch of the docs.

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[mautibot]

This issue has been mentioned on Mautic Forums. There might be relevant details there:

https://forum.mautic.org/t/post-thanksgiving-boost-contribute-to-mautic-this-open-source-friday/30038/1