Update plugin docs for new ORM and other changes.

Update the docs to reflect changes in the ORM. Clean up formatting and
a few other small issues. requestAction doesn't have the same
limitations as it used to have.
Add example of how TableRegistry should work with plugin tables.
Fix up docs based on review feedback.
Fix dash.
Merge pull request #1024 from cakephp/3.0-plugin

3.0 Update plugin docs