Improve documentation for WP-CLI commands

[GaryJones]

The WP-CLI commands for Co-Authors Plus are not particularly expressive for WP-CLI users.

Compare any of the DocBlocks against the WP-CLI 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.

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

[smistephen]

This documentation can also include unintuitive function behaviour, such as "the assign-coauthors command does not currently allow for multiple coauthors to be assigned".