Clarify the intent/meaning/behavior of @schema/type any=True in reference documentation

[pivotaljohn]

(Inspired by #515)

The reference documentation for @schema/type any=True does describe in general terms what this annotation means.
However, it does not make clear:

• how is the contained YAML treated (i.e. the fragment within the annotated node)?
• can I "break out" of this "any" mode by putting a @schema/type any=False deeper in the YAML tree?

The intention of @schema/type any=True is for ytt to treat it as ordinary YAML (potentially, but not likely templated).

• unlike in schema-declaring YAML, the contained fragment allows array values with any number of array items. (TODO: link to bug here)
• this also means that no array item defaulting occurs within the fragment.
• @schema/... annotations within this fragment is an error (TODO: link to enhancement request here)