Improve ActiveRecord::QueryMethods#includes docs

It's not immediately clear whether you can pass in multiple relations or
not. After going through the code a bit, I saw that the arguments are
just appended to an array. Also, added nested relations example.

[ci skip]
Merge pull request #11479 from henrikhodne/improve-ar-querymethods-includes-docs

Improve ActiveRecord::QueryMethods#includes docs [ci skip]