Clarify UI text on page for adding new content types

Issue Summary

This is to clarify the intro on admin/structure/types/add

Removed: "Each piece of content is of a specific content type."

(Why? They have just clicked 'Add content type' so that information isn't as useful here. Plus it's a slightly tricky sentence to understand)

Rephrased the next sentence: "Individual content types can have different fields, behaviors, and permissions assigned to it." to clarify that each content type is expected to be unique.

Added: "Save the new content type, then edit it to add fields."

(They cannot add new fields until the content type is saved)