Minor tidy-up of ShellCheck config

Author: ralish

.shellcheckrc

@@ -1,8 +1,10 @@
 # Add the script directory to the search path for source statements
 source-path=SCRIPTDIR
 
-# Enable all optional checks
+# Enable all checks (inc. optional)
 enable=all
 
-# Disable the require-variable-braces check
+# Disable some checks
+#
+# SC2250    require-variable-braces         optional
 disable=SC2250