Clarify that the `'header'` mark is only for internal usage (e.g., `_header`?)

The header mark is internally used to show title and subtitle.

gosling.js/src/gosling-schema/gosling.schema.ts

Lines 275 to 277 in c3169df

    
           // TODO: perhaps need to make this invisible to users 
        
           // being used to show title/subtitle internally 
        
           | 'header';

However, it is unclear if this is intended to be used by users or not.

We need to clarify that this is for the internal usage in the Gosling compiler.

	// TODO: perhaps need to make this invisible to users
	// being used to show title/subtitle internally
	\| 'header';