Mention template parameter vs. std:: (and make the limitation more generic...

Mention template parameter vs. std:: (and make the limitation more generic than necessary, to make it simpler to understand - it should involve default template arguments).


git-svn-id: http://root.cern.ch/svn/root/trunk@38317 27541ba8-7e3a-0410-8455-c3a389f83636