Improve 2019/burton/try.sh + shellcheck comments

The shellcheck comments are in cow.sh and check.sh: added the URL for
each supposed problem as per that issue.

2019/burton/check.sh

@@ -16,8 +16,11 @@ echo | $x
 /bin/echo -n a | $x
 echo a | $x
 $x < newlines
-# ^---^ SC2210 (warning): This is a file redirection. Was it supposed to be a comparison or fd operation?
-# Answer: 256 is a file so it is supposed to be file redirection.
+# 256 is a file so it is supposed to be file redirection. Thus we have to
+# disable this warning:
+#
+# SC2210 (warning): This is a file redirection. Was it supposed to be a comparison or fd operation?
+# https://www.shellcheck.net/wiki/SC2210
 # shellcheck disable=SC2210
 $x < 256
 $x < remarks