Add -Wdocumentation if supported by the compiler.
Fix two small documentation bugs.
Remove -Wno-deprecated-objc-isa-usage.

Inside object_{get,set}Class, a cast is used to get rid of the warning
now and all other direct isa references were removed.
Use __VA_ARGS__ for Block_copy.

This solves a corner case where the preprocessor would complain
otherwise.