Document behavior of trailing empty lines for `ExcessiveDocstringSpacing`

While beginning setup to look at branch coverage for ExcessiveDocstringSpacing, I found this behavior and thought we might document it.

In this example, it autocorrects trailing empty lines without flagging them.

Before submitting the PR make sure the following are checked:

[ ] Feature branch is up-to-date with master (if not - rebase it).
[ ] Squashed related commits together.
[ ] Added tests.
[ ] Updated documentation.
[ ] Added an entry to the CHANGELOG.md if the new code introduces user-observable changes.
[ ] The build (bundle exec rake) passes (be sure to run this locally, since it may produce updated documentation that you will need to commit).

If you have created a new cop:

[ ] Added the new cop to config/default.yml.
[ ] The cop is configured as Enabled: pending in config/default.yml.
[ ] The cop is configured as Enabled: true in .rubocop.yml.
[ ] The cop documents examples of good and bad code.
[ ] The tests assert both that bad code is reported and that good code is not reported.
[ ] Set VersionAdded: "<<next>>" in default/config.yml.

If you have modified an existing cop's configuration options:

rubocop / rubocop-rspec