Allow less strict attribute argument parsing

[nevans]

The prism parser is strict about attribute arguments: it only parses as an attribute when all arguments are symbols. rdoc 7.2's parser simply ignored any arguments after the initial symbol arguments.

As an example, the rdoc for Net::IMAP::Config intentionally took advantage of the looser parsing done by rdoc 7.2. That class redefines attr_reader, attr_writer and attr_accessor to add keyword arguments for type validation/coercion and defaults:

      # Seconds to wait until a connection is opened.
      #
      # Applied separately for establishing TCP connection and starting a TLS
      # connection.
      #
      # If the IMAP object cannot open a connection within this time,
      # it raises a Net::OpenTimeout exception.
      #
      # See Net::IMAP.new and Net::IMAP#starttls.
      #
      # The default value is +30+ seconds.
      attr_accessor :open_timeout, type: Integer, default: 30

rdoc 7.2 simply ignored the unknown keyword args, and parses this no differently from attr_accessor :open_timeout.

Fixes #1790.