Add undocumented about section fields (meta.yaml documentation)

[ytausch]

Description

The about section had some fields undocumented (most of them only in the example in the beginning of the documentation). These are added here.

Checklist - did you ...

• [x] Add a file to the news directory (using the template) for the next release's release notes?
• [x] Add / update necessary tests?
• [x] Add / update outdated documentation?

[conda-bot]

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[beeankha]

@conda-bot check