Handle ghost methods followed by undocumented methods

[p8]

Docs for ghost methods would get thrown away if the following method
does not have a comment. By parsing comments on linebreaks we avoid this
problem.

Builds on: #1258

[tompng]

This approach changes the way to write metaprogramming comment.
Example in https://ruby.github.io/rdoc/RDoc/Parser/Ruby.html#class-RDoc::Parser::Ruby-label-Metaprogrammed+Methods indicates that newline between metaprogramming comment and metaprogramming code is allowed, and this pull request changes it.

# This is OK
class ActiveStorage::Blob < ActiveStorage::Record
  ##
  # :method:
  #
  # Returns the associated ActiveStorage::Attachment instances.
  has_many :attachments, autosave: false
end

# OK in master, but document is not generated with this pull request
class ActiveStorage::Blob < ActiveStorage::Record
  ##
  # :method:
  #
  # Newline between metaprogramming comment and metaprogramming code

  has_many :attachments, autosave: false
end

The problem is in the RDoc notation ambiguity. ghost_method+undocumented_method and metaprogramming_comment+metaprogramming_code are ambiguous.
I found a workaround in rails/rails by inserting ##
https://github.com/rails/rails/blob/be9aa73dd72f1097be5d45a58d7912447a266bd1/activemodel/lib/active_model/attributes.rb#L90

##
# :method: ghost

##
private # this is not a metaprogramming code

Disallow newline is one possible approach but is a big change that need to be carefully considered.
Another possible approach is to disallow some specific pattern like def [different_name], private, public etc.
Or don't change and just document the workaround.

[p8]

Thanks for the explanation @tompng !
I'll try to add a test to document this behaviour.