Update the FIXME comment now that stream.clone is handled

lib/rspec/matchers/built_in/output.rb

@@ -235,10 +235,9 @@ def capture(block)
           # we're testing `is_a?(File)` in such an obnoxious way).
           inner_matcher = stream.to_io.is_a?(File)
 
-          # FIXME: stream.clone isn't sufficient right now - the StdErrSplitter
-          # clones with the same stream, and reopening the stream on either of
-          # them effectively updates both. Which means that the ensure-reopen
-          # further down doesn't really do anything currently.
+          # Careful here - the StdErrSplitter is what is being cloned; we're
+          # relying on the implemented clone method of that class (in
+          # rspec-support) to actually clone the File for ensure-reopen.
           original_stream = stream.clone
 
           captured_stream = Tempfile.new(name)