* Pull the definition of the extension in BIP342 to its own section
* Add a section to BIP341 on validating script path signatures
* Clarify that SigMsg does not produce the message being signed, but
a common portion of it
1. CHECKSIG / CHECKSIGADD is confused
Only the first OP-code for the first public key should be "CHECKSIG" and the following (second to n-th) OP-codes should be "CHECKSIGADD".
It is confusing because it is only specified the first and last OP-codes, so I specified the second OP-code clearly.
(I recommend to describe why only the first OP-code should be "CHECKSIG", not "CHECKSIGADD".)
2. Order of the signatures in witness
In the original sentence, the stack status after the all witness elements are pushed will be
| w_n |
| : |
| w_1 |
and then, the first element of the script, "<pubkey_1>" will be pushed to the stack
| pubkey_1 |
| w_n |
| : |
| w_1 |
so the "pubkey_1" and "w_n" won't match.
The order of either "pubkey_i"s or "w_i"s should be inverted.
