[glib] gcancellable: Mention nullability in g_cancellable_cancel() docs

[Philip Withnall <pwithnall src gnome org>]

commit e966cc51de22a9f8b045f6f0f6f0de82f934d46d
Author: Philip Withnall <philip withnall collabora co uk>
Date:   Tue Feb 24 10:57:14 2015 +0000

    gcancellable: Mention nullability in g_cancellable_cancel() docs
    
    Calling g_cancellable_cancel(NULL) is an explicitly allowed no-op, for
    convenience. Document and annotate that.

 gio/gcancellable.c |    6 ++++--
 1 files changed, 4 insertions(+), 2 deletions(-)
---
diff --git a/gio/gcancellable.c b/gio/gcancellable.c
index 258b88d..42785b2 100644
--- a/gio/gcancellable.c
+++ b/gio/gcancellable.c
@@ -458,7 +458,7 @@ g_cancellable_release_fd (GCancellable *cancellable)
 
 /**
  * g_cancellable_cancel:
- * @cancellable: a #GCancellable object.
+ * @cancellable: (nullable): a #GCancellable object.
  * 
  * Will set @cancellable to cancelled, and will emit the
  * #GCancellable::cancelled signal. (However, see the warning about
@@ -469,7 +469,9 @@ g_cancellable_release_fd (GCancellable *cancellable)
  * it from a thread other than the one running the operation that was
  * passed the @cancellable.
  *
- * The convention within gio is that cancelling an asynchronous
+ * If @cancellable is %NULL, this function returns immediately for convenience.
+ *
+ * The convention within GIO is that cancelling an asynchronous
  * operation causes it to complete asynchronously. That is, if you
  * cancel the operation from the same thread in which it is running,
  * then the operation's #GAsyncReadyCallback will not be invoked until