Update WP CLI command docs

[jeffpaul]

Description of the Change

This PR attempts to update our WP CLI docs in README.md as well as the comments in our commands themselves.

Alternate Designs

n/a

Benefits

Ensures those looking to leverage ClassifAI WP CLI commands have easy access to documentation on how to leverage these commands.

Possible Drawbacks

none identified.

Verification Process

Run the commands locally with the various options to see ClassifAI processing posts, text, and attacments.

Checklist:

• I have read the CONTRIBUTING document.
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my change.
• All new and existing tests passed.

Applicable Issues

n/a

Changelog Entry

Update docs (props @jeffpaul).

[dinhtungdu]

The h6 with code style of GitHub (the gray color) makes the parameter hard to read for me. Replacing h6 with b looks better for me.

https://github.com/10up/classifai/tree/update/wp-cli-docs

[jeffpaul]

@dinhtungdu sounds good, updated readme look good now?