Documentation: Clarify the behavior of parent when empty during block registration

[ellatrix]

Part of #41236.

Describe the bug

https://developer.wordpress.org/block-editor/developers/block-api/block-registration/#parent-optional

It might be good to update the documentation here to clarify that you can restrict it to the root if you pass an empty array. (Also good to check if there are tests for this or if it just happens to work.)

This also leads to the question what the default is for this parameter. undefined? null? In any case just "Type: Array" doesn't seem correct.

[cguntur]

@annezazu I would like to take this up but, I have a question. When I create a new block using inner blocks and set the parent to core/group, I can see that block when I want to add blocks to a group block, but otherwise I don't see it.

When I set the parent to an empty array [], I don't see the block at all in the editor. Am I missing something here?

Also, looks like the parent option only works when we use inner blocks. Is that correct?

I am just testing to make sure I am able to see the correct result before modifying the documentation.

[annezazu]

Amazing! Thanks for stepping in here @cguntur <3 @mkaz might you be able to help answer the above?