Format @return values the same as @param

Problem/Motivation

The type and description for return values is display in plain text.

Steps to reproduce

https://api.drupal.org/api/drupal/core%21lib%21Drupal%21Core%21Cache%21C...

Proposed resolution

Display the @return information in the same style as @parm

Remaining tasks

User interface changes

API changes

Data model changes