Clarify self?. method semantics

[soutaro]

It's a syntax for module function! 💡

This PR introduces two changes:

1. Singleton methods ignore public and private members: Singleton methods always have public visibility. Will be changed in Add public/private per member modifier #911
2. Instance methods defined with self? have private visibility, even with public

These changes will make self? methods closer to module function.

module Kernel
  def self?.puts: (*untyped) -> void     # Defines `Kernel.puts` public method and `Kernel#puts` private method.
end