Docs: Note cross-domain limitations for JWT token in readme/FAQ.

[peterwilsoncc]

Is your enhancement related to a problem? Please describe.

Add docs to clarify that the JWT token is:

• origin limited for environments intended to be online: development, staging and production
• does not have an origin limit for local environments

These values are configured by the WP_ENVIRONMENT_TYPE constant and definitions of how each environment is intended to be configured can be found in this make/core post.

Designs

N/A

Describe alternatives you've considered

N/A

Code of Conduct

• I agree to follow this project's Code of Conduct