Clarify difference and values of pipeline to API docs

What does this MR do?

There are two different API attributes for pipeline minutes in Users and Groups. The difference is currently not clear when reading the docs. To clarify this we will add information to make it clearer. Also adding information for possible values for shared_runners_minutes_limit.

Related issues

Belongs to https://gitlab.com/gitlab-org/customers-gitlab-com/-/issues/1549 as it came up while researching for that issue.

Author's checklist (required)

• Follow the Documentation Guidelines and Style Guide.
• If you have developer access or higher (for example, GitLab team members or Core Team members)
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When applicable:

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Review checklist

All reviewers can help ensure accuracy, clarity, completeness, and adherence to the Documentation Guidelines and Style Guide.

1. Primary Reviewer

• Review by a code reviewer or other selected colleague to confirm accuracy, clarity, and completeness. This can be skipped for minor fixes without substantive content changes.

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