lazutils: restored old CreateRelativePath, added doc

git-svn-id: http://svn.freepascal.org/svn/lazarus/trunk@41064 4005530d-fff6-0310-9dd1-cebe43e6787f
docs: CreateRelativePath does not support D:test for thread safety

git-svn-id: http://svn.freepascal.org/svn/lazarus/trunk@41065 4005530d-fff6-0310-9dd1-cebe43e6787f