Add ocamldoc 'since' tag to Set.find.

git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@13212 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02
Doc.

git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@13213 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02
Fix #5876.

git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@13214 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02