Improve comment on passThroughOptions

Fixes tj#1936 partially (docs need an update, too)

(cherry picked from commit b11e940)

lib/command.js

@@ -1535,7 +1535,9 @@ Expecting one of '${allowedValues.join("', '")}'`);
         }
       }
 
-      // If using passThroughOptions, stop processing options at first command-argument.
+      // If using passThroughOptions, stop processing options at first command-argument / unknown option.
+      // Processing after an unknown option is not possible because there is no way to know if the next argument is its option-argument and we should continue,
+      // or if it is a command-argument and we should stop.
       if (this._passThroughOptions) {
         dest.push(arg);
         if (args.length > 0) dest.push(...args);