Reworking abstract along NIST SP 800-63 and BCP taxonomy

[identitymonk]

I propose the following changes for the abstract to match more closely the objectives:

1. to provide guidance, and even more best current practices
2. to reach higher maturity by providing exact requirements for the assurance level targeted

This draft describes best current security practice for workload authentication and introduces the Workload Authentication Assurance Level (WAAL) framework that enables organizations to assess their current workload authentication mechanisms, implement the Best Current Practices (BCP) and progressively strengthen their security posture.