`Framing` section is not 100% accurate

[GitGab19]

While reviewing the framing_sv2 docs PR, I noticed that in the Framing chapter we are describing the msg_type field as "Unique identifier of the extension describing this protocol message".

This is not the correct description for sure, we need to fix it.

Also, I think that some concepts there are too much mixed, and hard to grasp.

[plebhash]

actually the description of extension_type is also bad... it says a bunch of things like "here's how you can repurpose bits from this field" without actually saying what this field is used for