Currently, the documentation for numSelectorArgs includes an incorrect
example. It shows a case where an argument of 1 will match a property
getter, but a getter will be matched only when N == 0.

This diff corrects the documentation and adds a test for numSelectorArgs(0).

Patch by Dave Lee.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@246998 91177308-0d34-0410-b5e6-96231b3b80d8