Clarify Pipeline subscriptions explanation

What does this MR do and why?

Clarify Pipeline subscriptions explanation

• "a new tag on the default branch" doesn't make sense: Tags don't have a connection to branches. Creating them "from a branch" is a shortcut but ultimately just specifies a SHA. So GitLab can't tell if a tag is "on the default branch" or not. We don't check/compare the SHA in this feature – any tag pipeline will trigger.
• "completes successfully" is misleading, even a failed or cancelled tag pipeline in the subscribed project will trigger.

See also !165499 (merged) – the docs themselves only had the second inaccuracy.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

Before	After

How to set up and validate locally

The changed text is in a project's CI/CD settings. (Requires Premium or higher license to show up.)