docs(sign): clarify description to include AMO upload

[vacuummirror]

Fixes #3493

Clarifies the sign command help text to reflect actual behavior: the command signs the extension and can upload it to Mozilla Add-ons (AMO). Current description implies local signing only. The CLI and docs support signing and uploading via AMO API keys. Clear wording reduces user confusion and matches README/help output.

[wagnerand-moz]

Thanks for the PR but this is not accurate. Signing is needed so the add-on can be installed in regular Firefox installations. The signing will happen on Mozilla's servers after uploading and approval.

[vacuummirror]

Thanks for the clarification. To align --help with README and the command reference:

Proposed help text:
“Submit the extension to Mozilla Add-ons (AMO) for signing so it can be installed in Firefox.”

Rationale from docs:

• For --channel=listed: creates a listing if needed and uploads a new version.
• For --channel=unlisted: uploads and returns a signed XPI.
• Signing occurs on Mozilla’s servers after upload/approval.

If you prefer shorter:
“Submit the extension to AMO for server-side signing.”

lmk if either of these work.

[wagnerand-moz]

Thanks!

@willdurand do you have a preference here?

[willdurand]

@willdurand do you have a preference here?

I do not, either is fine by me.

[vacuummirror]

I guess I'll go with the more detailed one: "Submit the extension to Mozilla Add-ons (AMO) for signing so it can be installed in Firefox."
Thank you so much!!!