resolve p field ambiguity and add signature disclaimer

Author: nGoline

11-payment-encoding.md

@@ -211,6 +211,10 @@ A writer:
 A reader:
   - MUST skip over unknown fields, OR an `f` field with unknown `version`, OR  `p`, `h` or
   `n` fields that do NOT have `data_length`s of 52, 52 or 53, respectively.
+    - MUST fail the payment if a mandatory field (`p` or `s`) was skipped due to an incorrect
+    length.
+    - MUST fail the payment if a `d` field is not present and a `h` field was skipped due to an
+    incorrect length. 
   - if the `9` field contains unknown _odd_ bits that are non-zero:
     - MUST ignore the bit.
   - if the `9` field contains unknown _even_ bits that are non-zero:
@@ -368,6 +372,7 @@ https://github.com/rustyrussell/lightning-payencode
 
 NB: all the following examples are signed with `priv_key`=`e126f68f7eafcc8b74f54d269fe206be715000f94dac067d1c04a8ca3b2db734`.
 All invoices contain a `payment_secret`=`1111111111111111111111111111111111111111111111111111111111111111` unless otherwise noted.
+Signatures are deterministic and generated using RFC6979 (using HMAC-SHA256). Note that even though using a `low R` would save 1 byte in the DER-encoded signature (by avoiding the need for a leading zero byte when the most significant bit is set), it is not enforced in this specification.
 
 > ### Please make a donation of any amount using payment_hash 0001020304050607080900010203040506070809000102030405060708090102 to me @03e7156ae33b0a208d0744199163177e909e80176e55d97a2f221ede0f934dd9ad
 > lnbc1pvjluezsp5zyg3zyg3zyg3zyg3zyg3zyg3zyg3zyg3zyg3zyg3zyg3zyg3zygspp5qqqsyqcyq5rqwzqfqqqsyqcyq5rqwzqfqqqsyqcyq5rqwzqfqypqdpl2pkx2ctnv5sxxmmwwd5kgetjypeh2ursdae8g6twvus8g6rfwvs8qun0dfjkxaq9qrsgq357wnc5r2ueh7ck6q93dj32dlqnls087fxdwk8qakdyafkq3yap9us6v52vjjsrvywa6rt52cm9r9zqt8r2t7mlcwspyetp5h2tztugp9lfyql