Add AMAU as a more defined option when talking about telemetry

Why is this change being made?

Action Monthly Active Users (AMAU) is a type of telemetry and it's important we are clear what we're talking about when talking about telemetry to avoid confusion.

Added the definition for AMAU as how it is defined in our handbook: https://about.gitlab.com/handbook/product/metrics/#action-monthly-active-users-amau

Does this MR meet the acceptance criteria?

Assign to DRI

• Did you assign this change to the correct DRI of the page or information you are changing?

Conformity

• Added description to this MR explaining the reasons for the proposed change, per say-why-not-just-what