Add missing Javadoc in clients/admin package

### Background
Under epic:

- #2092

### Split from:

- #2091

### Description
We'd like to add missing Javadoc for the clients/admin module.

### Analysis
We can start by removing the Checkstyle suppression for this, for both missingJavadocMethod and missingJavadocType.

Start by locating the clients/admin package and the Checkstyle configuration entries suppressing missingJavadocMethod and missingJavadocType. Remove those suppressions, add the required Javadoc throughout the affected package, and run the relevant Checkstyle or project validation to confirm the package passes without them.

Written by the indexing model from the issue text.