clarify extension_type field usage in message frames

[GitGab19]

This PR clarifies the usage of extension_type field in message frames, as discussed over discord.

cc @TheBlueMatt

[Shourya742]

Some nits I observed while reading the extension docs, can be taken up in this PR or some other PR.

1. unsupported type is a list:

   sv2-spec/extensions/extensions-negotiation.md

   Line 133 in 35ff875

   Client <--- RequestExtensions.Error [unsupported: 0x0002, required: []] ---- Server

   , we just show a value here, not list.

2. Both client extensions request are unsupported:

   sv2-spec/extensions/extensions-negotiation.md

   Line 141 in 35ff875

   Client <--- RequestExtensions.Error [unsupported: 0x0003, required: 0x0005] ---- Server

   , we have only shown one.

3. This needs to be changed:

   sv2-spec/extensions/worker-specific-hashrate-tracking.md

   Line 66 in 35ff875

   Client <--- RequestExtensions.Error [0x0002] ---- Server

   , not correct format of RequestExtension.Error

4. New line missing here:

   sv2-spec/extensions/worker-specific-hashrate-tracking.md

   Line 84 in 35ff875

   After negotiation, the client appends TLV fields at the end of `SubmitSharesExtended`.

5. We are not including the tlv inside the message, but outside its frame boundary:

   sv2-spec/extensions/worker-specific-hashrate-tracking.md

   Line 74 in 35ff875

   - The `user_identity` field **MUST** be included in `SubmitSharesExtended` as a TLV with Type `0x0002`.

   , this needs to be elaborated.