Accept misused :nodoc: usage

Usage of these :nodoc: are not correct but frequently misused. Maybe it's better to apply :nodoc: to the CodeObject next line.

class Foo

  # :nodoc: (This :nodoc: is applied to class Foo, not to method bar)
  def bar; end

  # :nodoc: (Same as above)
  def baz; end
end

Correct usage is:

class Foo
  # :nodoc: (for class Foo)

  def bar
  end

  def baz # :nodoc: (for method baz)
  end
end

This behavior looks like a bug.

class Foo

  # This is a comment for method bar
  # :nodoc: (this is a nodoc directive for class Foo)
  # :call-seq: bar(x, y) (this is a call-seq directive for method bar)
  # This is also a coment for method bar
  def bar
  end
end

Suggestion

If comment only contains container directive(including :nodoc:) and it appears at first line in class/module, it is a directive for container itself.

class Foo
  # :nodoc: (for container)
  # :stopdoc: (for container)

  def foo; end
end

If a comment before code object contains container directive(except :nodoc:), it's for container

class Foo
  ...

  # :stopdoc: (for container)

  def foo; end
end

Other comments are for code object. If it contains container directive(except :nodoc:), it's an error to be warned.

class Foo
  # :nodoc: (for method)
  # comment

  def foo; end

  # :nodoc: (for method)

  def foo; end
end

Directive categories

Directive categorization will be a bit complicated, but I think it's worth clarifying it.

Container directive

Must not mix with code object directives, comment containing these directives must not associated with code object.
:startdoc: :stopdoc: :enddoc: :doc: :category:

Container special directive

Must not mix with code object directives. Comment text is specially handled.
:section:

Situation dependent directive

Can be both container directive or code object directive
:nodoc:

CodeObject directive

Other directives. :callseq: :yield: and also unknown ones.

Use the Ruby examples in the issue as reproduction cases and trace RDoc's directive parsing and association behavior. Compare current handling with the proposed container, situation-dependent, and code-object categories, including warning behavior for mixed directives. Done means the examples resolve to the intended container or code object and invalid mixes are warned about.