Amend 'description' of the core-source 'Entity' class

[mbrush]

The description of the core-source Entity class was borrowed from the definition of the va-spec Entity class:

Entity is the root class of ‘core’ classes model - those that have identifiers and other general
metadata like labels, xrefs, urls, descriptions, etc. All core classes descend from and inherit
its attributes.

But Entity is used differently in the core-source model, and this definition not consistent its use here. (e.g. not all core-source.Entities have the general metadata types mentioned in the description above)

Note that we may want to resolve issue #110 concerning broader alignment of high level class structures between core-source and the va-spec, before finalizing the definition of the Entity class here.