Modified text structure, documented returning of NULL if there are no successful outcomes.

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/eventstudies@254 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
Added description of arguments, added return value for unscuccessful outcomes, changed function description and title.

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/eventstudies@255 edb9625f-4e0d-4859-8d74-9fd3b1da38cb