Document the non-strict-DER-conformance of one test in tx_valid.json.

In a signature, it contains an ASN1 integer which isn't strict-DER conformant due to excessive 0xff padding:
0xffda47bfc776bcd269da4832626ac332adfca6dd835e8ecd83cd1ebe7d709b0e

src/test/data/tx_valid.json

@@ -45,7 +45,7 @@
 "01000000010001000000000000000000000000000000000000000000000000000000000000000000006a473044022067288ea50aa799543a536ff9306f8e1cba05b9c6b10951175b924f96732555ed022026d7b5265f38d21541519e4a1e55044d5b9e17e15cdbaf29ae3792e99e883e7a012103ba8c8b86dea131c22ab967e6dd99bdae8eff7a1f75a2c35f1f944109e3fe5e22ffffffff010000000000000000015100000000", "P2SH"],
 
 ["The following is f7fdd091fa6d8f5e7a8c2458f5c38faffff2d3f1406b6e4fe2c99dcc0d2d1cbb"],
-["It caught a bug in the workaround for 23b397edccd3740a74adb603c9756370fafcde9bcc4483eb271ecad09a94dd63 in an overly simple implementation"],
+["It caught a bug in the workaround for 23b397edccd3740a74adb603c9756370fafcde9bcc4483eb271ecad09a94dd63 in an overly simple implementation. In a signature, it contains an ASN1 integer which isn't strict-DER conformant due to being negative, which doesn't make sense in a signature. Before BIP66 activated, it was a valid signature. After it activated, it's not valid any more."],
 [[["b464e85df2a238416f8bdae11d120add610380ea07f4ef19c5f9dfd472f96c3d", 0, "DUP HASH160 0x14 0xbef80ecf3a44500fda1bc92176e442891662aed2 EQUALVERIFY CHECKSIG"],
 ["b7978cc96e59a8b13e0865d3f95657561a7f725be952438637475920bac9eb21", 1, "DUP HASH160 0x14 0xbef80ecf3a44500fda1bc92176e442891662aed2 EQUALVERIFY CHECKSIG"]],
 "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", "P2SH"],