0x4261756D

[.NET] Fix VerifyUnion using wrong offset for typeId

2

The offset was overwritten to point to the data before the typeId was set.

Also added proper spec comments. Fixes at least one WPT test (168 subtests) that was failing previously: * https://wpt.live/encoding/single-byte-decoder.window.html?document

They are similar to their already existing decoder counterparts.