Added description of unspecified entities

[khieta]

Issue #, if available:

Description of changes:

• Added a couple sentences describing what an "unspecified" entity is.
• Updated the text about the appliesTo field in the schema since we recently discovered we were explaining it incorrectly (see rfc#35).

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.