Update the LangRef description of the 'returned' attribute

The description of the 'returned' attribute says that it is only used when
code-generating the caller. I'd like to make the optimizer smarter about
looking through functions with returned arguments (generally, but motivated by
my llvm.noalias work). As David pointed out in the review of D22202, the
LangRef should be updated to make its expanded uses clearer.

Differential Revision: http://reviews.llvm.org/D22205

llvm-svn: 275026