threads-docs: add a comment to the spawn article

basis/threads/threads-docs.factor

@@ -181,6 +181,7 @@ $nl
     "A simple thread that adds two numbers:"
     { $code "1 2 [ + . ] 2curry \"Addition thread\" spawn" }
     "A thread that counts to 10:"
+    ! Don't use $example below: it won't pass help-lint.
     { $code
       "USING: math.parser threads ;"
       "[ 10 iota [ number>string write nl yield ] each ] \"test\" spawn drop"