security: rename NO_SIGNATURE error code to MALFORMED_SIGNATURE

And move the documentation for all validation error codes to doxygen

Change-Id: Ibf4b507e3a544f2978192fbfad20cd7dc486818f
diff --git a/tests/unit/security/validation-policy-simple-hierarchy.t.cpp b/tests/unit/security/validation-policy-simple-hierarchy.t.cpp
index 38ab30c..788a7c9 100644
--- a/tests/unit/security/validation-policy-simple-hierarchy.t.cpp
+++ b/tests/unit/security/validation-policy-simple-hierarchy.t.cpp
@@ -43,7 +43,7 @@
 
   auto packet = Packet::makePacket(name);
   VALIDATE_FAILURE(packet, "Unsigned");
-  BOOST_TEST((lastError.getCode() == ValidationError::NO_SIGNATURE ||        // Interest
+  BOOST_TEST((lastError.getCode() == ValidationError::MALFORMED_SIGNATURE || // Interest
               lastError.getCode() == ValidationError::INVALID_KEY_LOCATOR)); // Data
 
   packet = Packet::makePacket(name);