Tiny fix: A top-level declaration pair (same basename) must be a getter and a setter

[eernstg]

Section 'Class Member Conflicts' specifies several conflicts (we can't have a method and a setter with the same basename, etc), but similar rules are missing for top-level declarations.

Only one of these rules is relevant (there are no top-level constructors so we can't have a conflict at the top level involving a constructor, etc.): It is an error to have a setter with basename n together with any other declaration with basename n, except when that other declaration is a getter.

This PR adds that rule.

Update: The rule in 'Class Member Conflicts' about setter/method conflicts was a bit vague: It referred to "member" with the intent that it should cover both static and instance members. It is now split into two sentences where one says 'the interface of $C$ contains an instance member' respectively '$C$ declares a static member'. The static/instance and instance/static cases are already covered by the previous sentence.

There is no associated implementation work, this behavior is already implemented.

[eernstg]

Thanks! Some changes were made in response to the review.