Attempt to not place ownership qualifiers on the result type
of block declarators.  Document the rule we use.

Also document the rule that Doug implemented a few weeks ago
which drops ownership qualifiers on function result types.

rdar://10127067

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@176336 91177308-0d34-0410-b5e6-96231b3b80d8