Mark optimizer CLI commands as experimental in their docblock

[schlessera]

Summary

This adds an (EXPERIMENTAL) suffix to the short descriptions of the following commands:

• amp optimizer optimize
• amp optimizer transformer list
• amp optimizer transformer config

It also adds the following note to each of these commands:

Note: The Optimizer CLI commands are to be considered experimental, as
the output they produce is currently not guaranteed to be consistent
with the corresponding output from the web server code path.

Example screenshot:

Related #6089

Checklist

• My code is tested and passes existing tests.
• My code follows the Engineering Guidelines (updates are often made to the guidelines, check it out periodically).

[github-actions]

Plugin builds for e377477 are ready 🛎️!

• Download development build
• Download production build