Expand text around abstract class LinkType

In earlier versions of PIM and in [WS-Calendar], [RFC5545], and [xCal] the LinkType is a UID, a URI, or a reference. The PIM creates an abstract class LinkType, for this purpose, and has reference as its only attribute.

This allows PSMs to easily define their Platform-Specific Relationship descriptions, and is a deliberate separation of abstraction from concrete references, and from Platform-Independent to Platform-Specific concerns. This technique acts as a hook for PSM implementeres.

Quoting from PIM CS01:

427 Note: In [WS-Calendar], [RFC5545], and [xCal] the LinkType is a UID, a URI [RFC3986], or a reference
428 string. This supports both distributed schedules and local identifiers that need not be fully qualified as
429 would be a UID or a URI. In the PIM, we use a string, without defining the precise type or uses of that
430 reference—that is left to the PSMs.

This text does not make clear why the abstract class is used, and the diagram notation is not addressed in the PIM specification. (See WSCALENDAR-557 and WSCALENDAR-551).