Port System.Runtime.Intrinsics source comments to Docs (#2359)

* Ported comments with workaround for issue with converted non-generic types.

* Resolve ambiguity for the "As" function.

Co-Authored-By: Ron Petrusha <[email protected]>

* Update Vector64.xml

* Update Vector256.xml

* Update Vector128.xml

* Update Vector64.xml

Author: carlossanlop