@defgroup should always have a topic

Problem/Motivation

Over in #2851678: rest.install and other .install files have malformed @defgroup doc blocks @jhodgdon spotted an issue with some @defgroups in core.

Proposed resolution

Ensure @defgorup has a topic ie. looks like

@defgroup updates-8.1.x-to-8.2.x Updates from 8.1.x to 8.2.x

Remaining tasks

User interface changes

API changes

Data model changes