Improve the documentation for Classes

[nicholascar]

In the Tern class hierarchy, these classes need better docstrings so they can be understood:

• Node
• Identifier
• Genid (none)
• RDFLibGenid (node)
• Variable

[aucampia]

Suggestion for Variable docstring here:

• Add a class diagram for terms. #1682 (comment)

[nicholascar]

I wonder what Genid and RDFLibGenid are even used for? I'll have to search the codebase as I've never come across them using RDFLib!

[aucampia]

I wonder what Genid and RDFLibGenid are even used for? I'll have to search the codebase as I've never come across them using RDFLib!

If memory serves, then one is for generating blank node identifiers and the other is not used, but I may be wrong about it and I can't recall which is which.

[edmondchuc]

I wonder what Genid and RDFLibGenid are even used for?

They are used for skolemization and de-skolemization of blank nodes. I remember writing a brief explanation in this PR #1493

[nicholascar]

Thanks for the reminder @edmondchuc!