15.2 Revisit partial class statement position

Split off from ECMA-TC49-TG2/spec#751, where we decided to make all clauses consistent for C# 5 with minimal changes.

Each clause is therefore:

- Description of what makes a class Foo (where Foo is abstract/sealed/static)
- Description of what that means
- Statement about when partial classes are Foo

It feels like the final statement should be part of the first part, describing when the condition applies. Alternatively, there could be a blanket statement at the end of 16.1.