Merge "docs: Updated Billing API documentation (for Project Nickel)." into mnc-io-docs am: 5ea88aad9d
am: 1ee7930329

* commit '1ee7930329356b0748ae7b49cc4d89b2874cc98c':
  docs: Updated Billing API documentation (for Project Nickel).

Change-Id: I695fb2c4a7f0c80148657df6bd3158ef327a6212