Add note about `wp admin` being a separate package

[hirasso]

Feature Request

• [x] Yes, I reviewed the contribution guidelines.

Describe your use case and the problem you are facing

The page about wp admin only documents the command, but running wp admin won't work without first installing the command.

Describe the solution you'd like

The page could provide a hint that the command needs to be installed separately, as described here: https://github.com/wp-cli/admin-command/

[hirasso]

Suggestion:

This command must be installed before you can use it: https://github.com/wp-cli/admin-command/#installing

If that wording looks good to the maintainers, I'd happily create a PR :)

[swissspidy]

There's already a section about installing on that page:

Are you suggesting adding an additional sentence to that section for commands like this which aren't bundled with WP-CLI by default?

[hirasso]

OMG, you are right! 🫣😄 ...somehow I assumed that the global parameters would be the last bit of information on the page, so I stopped scrolling.

What would you think about moving the "Installing" section up on the page, so that it appears before the global parameters?

[hirasso]

I'll close this as it's kind of a non-issue