Use inherited documentation when checking alias parameter coverage

[sjh9714]

Summary

Fixes #1567.

rdoc -C1 lists the parameters of an aliased method as undocumented, even
though the alias inherits the original method's documentation. rdoc -C on the
same file already reports it as fully covered.

Given the file from the issue:

##
# Instance methods

module InstanceMethods
  ##
  # Takes a +value+ or a +block+ and returns a value monad.

  def _ value = nil, &block
    # ...
  end

  alias value  _
  alias expect _
end

rdoc -C1 reports:

The following items are not documented:
  Method:
    InstanceMethods#expect
      Undocumented params: value, block
    InstanceMethods#value
      Undocumented params: value, block

Methods:     3 (0 undocumented)
Parameters:  6 (4 undocumented)

Methods: 3 (0 undocumented) is the tell: the aliases already count as
documented, but their parameters do not.

RDoc::MethodAttr#documented? resolves inherited documentation through
#is_alias_for and #see, while RDoc::Stats#undoc_params parses
method.comment directly, which is empty for an alias, so every parameter comes
back undocumented.

Changes

• RDoc::Stats#undoc_params falls back to the comment of #is_alias_for or
  #see when the method carries no comment of its own. A method with its own
  comment is unaffected, so the only behavior change is for objects
  #documented? already treats as documented.
• Regression test alongside the existing test_undoc_params_* cases.

Testing

• ruby -Ilib -Itest test/rdoc/rdoc_stats_test.rb -n /undoc_params/ — the new
  test fails without the fix (<["a"]> was expected to be empty) and passes
  with it.
• rake normal_test — 2520 tests, 6165 assertions, 0 failures, 0 errors.
• bundle exec rubocop lib/rdoc/stats.rb test/rdoc/rdoc_stats_test.rb — no
  offenses.
• ruby -Ilib exe/rdoc -C1 --op /tmp/rdocout /tmp/rdoc_bug.rb on the file above
  now reports Parameters: 6 (0 undocumented) and 100.00% documented.