Added "and/or brightness (e.g. monochromatic)" to the Random Effect description

[GitHubGoody]

The Random Effect can also affect the brightness instead of color (e.g. monochromatic light components). I'm not sure if the brightness is also affected for RGB light components, so I put it as an "and/or" instead of just an "or." Of course, adjust based on how it actually works with RGB lights.

Description:

Related issue (if applicable): fixes

Pull request in esphome with YAML changes (if applicable): esphome/esphome#<esphome PR number goes here>

Checklist:

• [ ] I am merging into next because this is new documentation that has a matching pull-request in esphome as linked above.
  or

• [X] I am merging into current because this is a fix, change and/or adjustment in the current documentation and is not for a new component or feature.

• [ ] Link added in /index.rst when creating new documents for new components or cookbook.