make clean: comment about other targets

git-svn-id: http://svn.freepascal.org/svn/lazarus/trunk@44124 4005530d-fff6-0310-9dd1-cebe43e6787f
IDE: make distclean, cleanlaz: delete all output directories ../units/*

git-svn-id: http://svn.freepascal.org/svn/lazarus/trunk@44125 4005530d-fff6-0310-9dd1-cebe43e6787f