Clarify role of abstract_archival_object schema

[lorawoodford]

Minor tweak to clarify that abstract_archival_object is the parent schema for the four major archival record types: resources, archival objects, digital objects, and digital object components.

Description

Also includes small tweak to remove duplicative information from digital object component schema.

Related JIRA Ticket or GitHub Issue

Motivation and Context

It was not obvious by naming convention that this abstract schema applied to the four major record types.

How Has This Been Tested?

Confirmed no changes were noticeable within the application.

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.