Improve the documentation (bis) with Arthur and Chandler's comments

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@194412 91177308-0d34-0410-b5e6-96231b3b80d8

docs/tools/clang.pod

@@ -304,13 +304,9 @@ Like B<-Os> (and thus B<-O2>), but reduces code size further.
 
 Equivalent to B<-O2>.
 
-=item B<-O4>
+=item B<-O4> and higher
 
-Equivalent to B<-O3>.
-
-=item B<-O5> and upper
-
-Fall back on B<-O3>.
+Currently equivalent to B<-O3>
 
 =back