update ld2420 documentation to include optional uart_id and ld2420_id configuration variables

[nkinnan]

Description:

update ld2420 documentation to include optional uart_id and ld2420_id configuration variables

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.