Define a term used several times.
Fix various bits of rst formatting.
Fix some typos and improve consistency of capitalisation etc.
Fix an issue with the interpretation of units.
Minor clarifications of various rules.

This included splitting some rules into multiple sub-rules, adding an extra
sub-rule on occasion, being explicit about attribute namespaces, and the like.
Fix the rules around relationship_ref.

Notably, the name attribute was not mentioned when this element was defined.
Add a rule to prevent import cycles.
Clearer wording prohibiting units reference cycles.
Merge pull request #9 from jonc125/master

More changes from proofreading all the rules