Add wording to explain additional minutes denominator change

Problem to solve

Clarify for users how the denominator is calculated for additional pipeline minutes.

Intended users

All .com users/groups who purchase additional minutes

Further details

Each month (when pipeline minutes are reset), the denominator of the additional minutes will change based on a calculation. The "total" now available will be: purchased + unused additional (from previous months)

Proposal

Add a brief explanation to the appropriate customer docs section, and a link to the doc.

Next to the base usage minutes, we have a little ? icon that links to the relevant documentation. We could do something similar for additional minutes.

What does success look like, and how can we measure that?

Customers are no longer confused about why the denominator of the additional minutes change.