Add a top-level overview to the apt mirror API documentation.

Clarify that when AnnotationValue.getValue() returns a Collection,
elements are of type AnnotationValue. This is actually a bug in
the HTML formatting of the doc comment: need to use @code instead
of <code>.

Replace most references to "apt" with vaguer references to an
"annotation processing tool". Although the com.sun.mirror.apt
package is tailored quite closely to the apt tool, it could -- in
theory at least -- be wholly or partially used by other tools.

Add missing description to DeclarationFilter default constructor.