[Doclet API] address isVisible() and shouldDocument()

These methods are needed by the doclet, and are javadoc/doclet specific.

isIncluded():
an element is included if,
    - included lists contains the element
    - shouldDocument is true

shouldDocument():
An element must be documented if:
   - the element is not SYNTHETIC
   - allowed by the user's selection filtering (ModifierFilter)

   if TYPE then
       - not SYNTHETIC
       - excluded non source types (classes-path vs source-path)
       - can be documented (isVisible)

isVisible():
A type element is said to be visible (could be documented) if:
    - allowed by the user's selection filtering (ModifierFilter)
    - a nested type then, the enclosing type is:
      - STATIC
      OR
      - could be documented ie. visible

We need to incorporate isVisible and shouldDocument in DocletEnvironment,
it may be possible to fold the capability of isVisible completely into shouldDocument
thereby eliminating the need to expose isVisible.