Added comment about the need for the ISVNSession.

In response to https://github.com/jenkinsci/subversion-plugin/commit/3f09b1c6c0402235e1e4b8719a134f49d9c40766#commitcomment-5152466

Due to the caching issues that lead me to write #60 later, all ~7k jobs in my instance opened at least one connection to SVN. This was heavily clogging the tubes at the SVN server. With this change, the problem went away.