Clarifies applicability of container constraints and open content

[popematt]

Issue #, if available:

Fixes #88 Fixes #5

Description of changes:

Clarifies applicability of some container constraints in Ion Schema 1.0 specification

• Specifies that element also applies to document
• Specifies that contains also applies to document
• Moves content constraint to be in the "Struct Constraints" section instead of "Container Constraints" section.

...and updates open content section to only refer to structs since it only actually applies to structs.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.