Clarified isClass() by adding:

 * This includes errors and exceptions but not interfaces.

Added terminology section to package.html and clarified all 
program elements that contain the word "class".

No regression test included because this affects only the doc comment.

 xxxxx@xxxxx  2002-09-29



The RootDoc.classes() and PackageDoc.allClasses() mean
by "class" -- an interface, error, exception, or ordinary
class.

Doc.isClass() returns true if the item is a
"class", but not an interface. (It's not clear whether it
includes errors and exceptions, or not.)

Throughout the API, the word "class" needs to be clearly described
as to whether it means "class and interface" or "class, not interface".
======================================================================

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If we want clarity I suggest renaming:
  RootDoc.classes() to RootDoc.classesAndInterfaces()
  PackageDoc.allClasses() to allClassesAndInterfaces()

 xxxxx@xxxxx  2002-09-29