[xcm-v3] trait DispatchBlob return value issue

[bkontur]

originally reported here: #4097 (comment)
(but probably lost in lots of comments in xcm-v3 big PR)

Isssue:
misleading doc and returns the unexpectable weight consumed by the dispatch vs Result<() ?

        /// Dispatches an incoming blob and returns the unexpectable weight consumed by the dispatch.
	fn dispatch_blob(blob: Vec<u8>) -> Result<(), DispatchBlobError>;

https://github.com/paritytech/polkadot/blob/master/xcm/xcm-builder/src/universal_exports.rs#L235-L237

Proposed solution:

1. fix doc - just remove and returns the unexpectable weight consumed by the dispatch
2. fix return value, like:

   fn dispatch_blob(blob: Vec<u8>) -> Result<Weight, DispatchBlobError>
   or
   fn dispatch_blob(blob: Vec<u8>) -> Result<Option<Weight>, DispatchBlobError> 
   

3. other?

[bkontur]

@gavofyork @KiChjang if you have sometime, could you, please, look at this? I could fix it, if you give me some hints :)

[bkontur]

fixed docs by #7230