rewrites AR's reload documentation

* It is no longer true that options are passed to find.

* Documents that ActiveRecord::RecordNotFound may be raised.

* Documents that the reload happens in-place.

* Documents that caches are wiped.

* Documents that the method returns self.

* Documents a couple of use cases.
revises the documentation of ActiveRecord::Relation#find_or_create_by [ci skip]

* Inspect uses double quotes.

* Inspect puts a hash as in #<User ...>.

* Documents the return value, and makes explicit it can be an invalid record.

* Documents the method is not atomic.

* Documents a way to handle UNIQUE contraint violations in the event of a race condition.

* Removes the "Examples" header according to our guidelines.