Clarifies additional runner minutes balance change

What does this MR do?

In #3314 (closed) we describe how the denominator changes month to month as:

The additional minute count will show the total available for use this month. This means that every month, the count will be readjusted to remove used minutes. For example:

• February 15: A group buys 4000 minutes. The count reads 0/4000 minutes.
• February 28: The group has used 1500 minutes. The count reads 1500/4000 minutes.
• March 1: The counter reads: 0/2500 minutes.

As a user, this is a bit confusing (https://gitlab.zendesk.com/agent/tickets/140217) so we should note the behviour in the docs.

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

Security

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• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
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