Fixes 595 in ember-learn/ember-api-docs - missing has-block API documentation

[habdelra]

This fixes issue: ember-learn/ember-api-docs#595

I think there are a bunch of helpers that are more foundational to glimmer, like {{yield}} and {{with}} where the documentation was just put in the packages/@ember/-internals/glimmer/index.ts, despite the real functionality living deeper below. This seems to fall into that bucket, which probably explains why the documentation is missing from the API docs. The clearest place where I could find where the {{has-block}} helper is handled is:

https://github.com/emberjs/ember.js/blob/master/packages/%40ember/-internals/glimmer/lib/component-managers/curly.ts#L239-L242

But really, I think there are a lot of places that actually contribute to this functionality. Probably documenting {{has-block}} in the packages/@ember/-internals/glimmer/index.ts file is the most straightforward place to put it.

[habdelra]

This also now addresses this issue: ember-learn/ember-api-docs#594 where we are now adding documentation for the {{has-block-params}} helper.

[rwjblue]

Looking really good! Could you also add examples using angle bracket invocation?

<MyComponent /> {{! without block }}
<MyComponent></MyComponent> {{! with block }}

Also, has-block accepts a single argument: the name of the block. The name (at the moment) is either "main" or "inverse" (though only curly components support inverse). I think that argument needs to be added to the docs for both has-block and has-block-params.

[rwjblue]

Thank you!!!