ux: clarify Prepare release workflow inputs

[vitormattos]

Improves the maintainer-facing workflow_dispatch form without adding extra inputs or changing release behavior.

• removes redundant Optional wording;
• explains the effect of ref/version/channel/backport/milestone/security inputs in one short sentence each;
• adds one link to the focused release guide instead of repeating documentation links;
• adds structural coverage so the form does not regress to vague descriptions.

No imported Nextcloud workflow is changed.