[glibmm] Gio::PollableInputStream, PollableOutputStream: Fix a comment

[Kjell Ahlstedt <kjellahl src gnome org>]

commit 1fb88282f37e753b351ceaefdbfeeb3fe9c9d93c
Author: Kjell Ahlstedt <kjellahlstedt gmail com>
Date:   Sun Jun 18 18:42:10 2017 +0200

    Gio::PollableInputStream, PollableOutputStream: Fix a comment
    
    Make it clear that Pollable[In|Out]putStream can't be derived from
    [In|Out]putStream even though GPollable[In|Out]putStream requires
    G[In|Out]putStream. Bug 776537

 gio/src/pollableinputstream.hg  |   10 +++++-----
 gio/src/pollableoutputstream.hg |   10 +++++-----
 2 files changed, 10 insertions(+), 10 deletions(-)
---
diff --git a/gio/src/pollableinputstream.hg b/gio/src/pollableinputstream.hg
index a5dcddf..ada354d 100644
--- a/gio/src/pollableinputstream.hg
+++ b/gio/src/pollableinputstream.hg
@@ -30,11 +30,11 @@ namespace Gio
 
 class Cancellable;
 
-//TODO: Instead derive from InputStream, when we can break ABI,
-//because the GPollableInputStream interface requires the GInputStream interface.
-//LoadableIcon does a similar thing correctly, for instance.
-// However, GInputStream is an actuall class, deriving from GObject,
-// but GPollableInputStream is a GInterface, that "requires" GInputStream.
+// GPollableInputStream requires GInputStream, but Gio::PollableInputStream
+// can't be derived from Gio::InputStream, because Gio::InputStream is a
+// Glib::Object. Gio::LoadableIcon is a different case. It can be derived from
+// Gio::Icon, because both are interfaces.
+// See https://bugzilla.gnome.org/show_bug.cgi?id=776537
 
 /** PollableInputStream - Interface for pollable input streams.
  * PollableInputStream is implemented by InputStreams that can be polled for
diff --git a/gio/src/pollableoutputstream.hg b/gio/src/pollableoutputstream.hg
index 2ab20cb..23a127c 100644
--- a/gio/src/pollableoutputstream.hg
+++ b/gio/src/pollableoutputstream.hg
@@ -30,11 +30,11 @@ namespace Gio
 
 class Cancellable;
 
-//TODO: Instead derive from OutputStream, when we can break ABI,
-//because the GPollableOutputStream interface requires the GOutputStream interface.
-//LoadableIcon does a similar thing correctly, for instance.
-// However, GOutputStream is an actual class, deriving from GObject,
-// but GPollableOutputStream is a GInterface, that "requires" GOutputStream.
+// GPollableOutputStream requires GOutputStream, but Gio::PollableOutputStream
+// can't be derived from Gio::OutputStream, because Gio::OutputStream is a
+// Glib::Object. Gio::LoadableIcon is a different case. It can be derived from
+// Gio::Icon, because both are interfaces.
+// See https://bugzilla.gnome.org/show_bug.cgi?id=776537
 
 /** PollableOutputStream - Interface for pollable output streams.
  * PollableOutputStream is implemented by OutputStreams that can be polled for