-Wdocumentation should not check the @returns command for Objective-C

function/block pointer properties

The commit r300981 allowed @param/@return commands for function/block
pointer property declarations. This meant that -Wdocumentation started warning
about @return that was used to document properties whose function/block type
returned void. However, prior to that commit, we allowed @return for all
property declarations, because it can be used to document the value that's
returned by the property getter. This commit restores the previous behaviour:
now the @return command can be used to document all properties without warnings.

rdar://24978538

llvm-svn: 301402