Updated coding-guideline documentation to include experimental APIs

[abotteram]

Description

Adds documentation to coding-guideline.md, regarding experimental exposed APIs.

Experimental APIs

Exposed APIs that are still being tested, discussed and are subject to change should be prefixed with __experimental, until they are finalized. This is meant to discourage developers from relying on the API, because it might be removed or changed in the (near) future.

Example:

export {
	internalApi as __experimentalExposedApi
} from './internalApi.js';

How Has This Been Tested?

Screenshots (jpeg or gifs if applicable):

Types of changes

Documentation

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code has proper inline documentation.

[atimmer]

For clarification, I would change the imported API to internalApi. So the code example becomes:

export {
	internalApi as experimental__exposedApi
} from './internalApi.js';

[aduth]

Pending feedback from #4484 (comment) per how the demonstrated example is an ESLint error on camelcase

[aduth]

Nice 👍