[docs] Document how an `import` may use a tag

[aureq]

What happened?

When reading the ESC reference page, it's not clear to the user they can actually import a separate environment with a specific tag.

The doc reads

imports:
  - environment-a
  - environment-b

But this would be more useful (as an example)

imports:
  - environment-a
  - environment-b:prod

Also, is it possible to import a specific revision using @X where X is a revision number?

Example

n/a

Output of pulumi about

n/a

Additional context

n/a

Contributing

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