Improve EntityOwnerInterface return value documentatio

Problem/Motivation

#2078387: Add an EntityOwnerInterface introduced \Drupal\user\EntityOwnerInterface, of which the setter methods are documented to return self instead of $this.

Proposed resolution

Update @return self to @return $this.

Remaining tasks

None.

User interface changes

None.

API changes

No practical changes, as this was the intended behavior anyway.