[gimp] app: improve GimpAsync description

[N/A <ell src gnome org>]

commit 729240db34d5e1d1558fc160705611ab9dacbe67
Author: Ell <ell_se yahoo com>
Date:   Sat May 12 08:49:42 2018 -0400

    app: improve GimpAsync description

 app/core/gimpasync.c |    4 +++-
 1 files changed, 3 insertions(+), 1 deletions(-)
---
diff --git a/app/core/gimpasync.c b/app/core/gimpasync.c
index ec1d34a..692554b 100644
--- a/app/core/gimpasync.c
+++ b/app/core/gimpasync.c
@@ -43,7 +43,9 @@
  * and the "async thread" is the thread calling 'gimp_async_finish()' or
  * 'gimp_async_abort()' (which may be the main thread).  The main thread is
  * said to be "synced" with the async thread if both are the same thread, or
- * after calling 'gimp_async_wait()' on the main thread.
+ * after the execution of any of the callbacks added through
+ * 'gimp_async_add_callback()' had started, or after calling
+ * 'gimp_async_wait()' on the main thread.
  */