Refactor MemberRole#base_access_level

What does this MR do and why?

• Follow up based on MR feedback, issue: #420061 (closed)
• The API description is for documentation only and does not affect API behavior: https://github.com/ruby-grape/grape#describing-methods
• This also adds a validation for base_access_level at the model level so that we are validating this at both the API level and model level.

How to set up and validate locally

In a rails console, try:

MemberRole.create!(base_access_level: 1)

And you should see a validation error.

All other updates here are documentation only.