Docs: Removed internal is* properties.

[mrdoob]

I don't think we should document these.

[donmccurdy]

Hm, do you disagree with my comment at #18195 (comment)? I don't necessarily feel strongly that the properties need to be documented, but I hadn't personally considered them "internal", and I definitely prefer to use them in application code versus .type === "Mesh".

Ideally foo instanceof Mesh would just work, but was there some issue with that? I can't remember why we don't use it internally and use isFoo instead.

[mrdoob]

I do not disagree.
You can use them but language like "default is true" in the documentation was confusing.

[mrdoob]

If we're going to document them, we can automate it with .is[name].
But I don't think I would call it a Boolean as it makes you think you can change it.

[donmccurdy]

Ok, thanks — I don't mind removing it from the API documentation.