docs: minor clarity edits on "Available Keys" section; platforms supported

[bkreider]

Checklist

• [X] I added a descriptive title
• [X] I searched open reports and couldn't find a duplicate

What happened?

I think this could be put into a "Note box". It is also a little confusing at first for anyone that doesn't know conda-build.

Note that the special environment variables available in meta.yaml when running conda-build are not available here.

Can the documentation add a field platform for the keys that have restrictions on the platform?

Name required: yes type: string

Conda Info

No response

Conda Config

No response

Conda list

No response

Additional Context

No response

[jaimergp]

I want to tackle this as part of a migration of the config system to a pydantic model.