Improve documentation for WP-CLI commands

The WP-CLI [commands](https://github.com/Automattic/Co-Authors-Plus/blob/master/php/class-wp-cli.php#L118) for Co-Authors Plus are not particularly expressive for WP-CLI users.

Compare any of the DocBlocks against the [WP-CLI Documentation Standards](https://make.wordpress.org/cli/handbook/documentation-standards/), and it's clear that more could be done:

- OPTIONS sections; describes the options in more detail than the `@synopsis`.
- EXAMPLES sections; many visual learners understand something better be reading examples and adapting to their needs.

The commands class has no documentation at all, let alone the suggested [class annotation](https://make.wordpress.org/cli/handbook/documentation-standards/#class-annotation).

Extra reference: https://make.wordpress.org/cli/handbook/commands-cookbook/#annotating-with-phpdoc

Start in php/class-wp-cli.php around the Co-Authors Plus command definitions and compare each existing DocBlock with the WP-CLI Documentation Standards. Review the commands cookbook and document the command class and its commands with the requested class annotation, detailed OPTIONS sections, and EXAMPLES sections. Done means the WP-CLI commands are consistently documented for users.

Written by the indexing model from the issue text.