Merge #16885: doc: Update tx-size-small comment with relevant CVE disclosure

c4b0c08f7c Update tx-size-small comment with relevant CVE disclosure (Gregory Sanders)

Pull request description:

  Code first introduced under https://github.com/bitcoin/bitcoin/pull/11423 with essentially no description and no discussion.

ACKs for top commit:
  MarcoFalke:
    ACK c4b0c08f7c
  fanquake:
    ACK c4b0c08f7c

Tree-SHA512: 95d5c92998b8b1e944c477dbaee265b62612b6e815099ab31d9ff580b4dff777abaf7f326a284644709f918aa1510412d62310689b1250ef6e64de7b19ca9f71

test/functional/data/invalid_txs.py

@@ -99,6 +99,8 @@ class InputMissing(BadTxTemplate):
         return tx
 
 
+# The following check prevents exploit of lack of merkle
+# tree depth commitment (CVE-2017-12842)
 class SizeTooSmall(BadTxTemplate):
     reject_reason = "tx-size-small"
     expect_disconnect = False