Clarify description of `--alias` flag for `deploy` command

Is your feature request related to a problem? Please describe.

We got some docs-related feedback about the --alias flag for the deploy command.

The alias flag was renamed to from the branch flag, and the branch flag is now deprecated. Your explanation makes sense if you think about it as if the flag is named branch because then I could say “well I’m deploying the branch which I have defined as my production branch so I expect it to go to prod”. But if the flag is named alias and the documentation says that this is useful in order to create a more predictable preview URL, then the association between that and what I chose to be the production branch is gone.

Essentially they’re still an implicit connection between “alias” and git branches.

Describe the solution you'd like

Additional content or edits to the deploy documentation, specifically the --alias description.

Additional context

Support Forums post with details about the confusion

Can you submit a pull request?

Yes (if provided more information)

Start with the deploy documentation linked in the issue and locate the --alias description. Read the linked support-forum context to clarify the remaining connection with Git branches, then update the explanation so its behavior and purpose are unambiguous; the deploy docs should clearly explain what the flag does and why it affects preview URLs.