Docs: Fix return value type of BufferAttribute.onUpload

[luisfonsivevo]

The method BufferAttribute.onUpload returns this (the BufferAttribute itself), not null. There are also many other BufferAttribute methods that return this, but they are all marked as returning BufferAttribute. I'm not sure if this was intentional, but the onUpload method return value type is definitely incorrect in the docs. I can update the PR (and Chinese docs) once clarified.

[Mugen87]

There are also many other BufferAttribute methods that return this, but they are all marked as returning BufferAttribute.

I would say this is incorrect. E.g. clone() has a return type of BufferAttribute since it creates a new instance. Whenever a method returns this, it should be this in the docs and TS declaration files, too.

[mrdoob]

Thanks!