Clarify that references to inner elements only apply to JSON Schema / XSD

[jstoiko]

The References to Inner Elements section implies that this only applies to JSON and XML schemas, but it's not clear enough.

Proposed clarification

• change the sentence:

When referencing an inner element of a schema.

to read:

When referencing an inner element of a JSON or XML schema.

• add a reference to the JSON pointer RFC[1] for references to inner elements of JSON Schemas, i.e. at the end of the list of restrictions add:

• References pointing to inner elements of JSON schemas MUST be valid JSON Pointers as defined in RFC6901

[1] https://tools.ietf.org/html/rfc6901