ISSUE-216: How to explain syntax rules for ill-formed shapes

I am trying to find a format for our spec that is both user-friendly and 
sufficiently formal. I believe Peter's prose is aimed at a formal 
audience only.

A notable difference between the syntax proposed by Peter and the 
current spec is that he makes it more explicit which triples lead to 
ill-formed shapes graphs. For example (from Dimitris' draft) - I hope 
the screenshots get through:





So it's basically formulated in the direction of what are ill-formed 
nodes, not in the direction of guiding the user of what values are 
actually expected.

On the restructuring branch, I have added a grey box as syntax 
highlighting around the formal syntax rules, and the preamble of the 
document explains that violations of these rules lead to ill-formed graphs:



...

(Also note that Peter's proposal dropped the explanatory prose that 
Karen explicitly requested, and added a new rule that would disallow 
false as a value for sh:deactivated - this change was never discussed or 
agreed upon).

Holger