update AMAU definition

Why is this change being made?

Per the Growth GC, Sid mentioned that AMAU is more than just a metric thats used by SMAU. Removed 'Actions Monthly Active Users is a KPI that is required to calculate SMAU' and added that its used to measure the success of features.

Does this MR meet the acceptance criteria?

Conformity

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