- toString -> getShortDescription so that ParameterValue.getShortDescription() can explain what this method is expected to return.
- added proper HTML escape to avoid XSS vulnerability.
- modified to use StringBuilder for string concatanation.

git-svn-id: https://hudson.dev.java.net/svn/hudson/trunk/hudson/main@21252 71c3de6d-444a-0410-be80-ed276b4c234a