[glib] docs: Mention the idiomatic way of invoking a GAsyncReadyCallback

[Debarshi Ray <debarshir src gnome org>]

commit 7339a09376dfe39934e7f59b8760dbb7ebb731cd
Author: Debarshi Ray <debarshir gnome org>
Date:   Fri Jun 16 14:47:33 2017 +0200

    docs: Mention the idiomatic way of invoking a GAsyncReadyCallback
    
    The GAsyncResult documentation didn't specify the context in which the
    GAsyncReadyCallback is expected to be invoked. Since asynchronous
    operations can be implemented in various ways involving GSources,
    threads and coroutines, it is useful to mention what the standard
    expections are.
    
    Unfortunately, since this was left undefined for so long, we can only
    phrase it as a suggestion, and not as a hard requirement.
    
    https://bugzilla.gnome.org/show_bug.cgi?id=783825

 gio/gasyncresult.c |   17 ++++++++++-------
 1 files changed, 10 insertions(+), 7 deletions(-)
---
diff --git a/gio/gasyncresult.c b/gio/gasyncresult.c
index cffe889..b96f1ee 100644
--- a/gio/gasyncresult.c
+++ b/gio/gasyncresult.c
@@ -36,13 +36,16 @@
  * which are chained together by a #GAsyncReadyCallback. To begin
  * an asynchronous operation, provide a #GAsyncReadyCallback to the
  * asynchronous function. This callback will be triggered when the
- * operation has completed, and will be passed a #GAsyncResult instance
- * filled with the details of the operation's success or failure, the
- * object the asynchronous function was started for and any error codes
- * returned. The asynchronous callback function is then expected to call
- * the corresponding "_finish()" function, passing the object the
- * function was called for, the #GAsyncResult instance, and (optionally)
- * an @error to grab any error conditions that may have occurred.
+ * operation has completed, and must be run in a later iteration of
+ * the [thread-default main context][g-main-context-push-thread-default]
+ * from where the operation was initiated. It will be passed a
+ * #GAsyncResult instance filled with the details of the operation's
+ * success or failure, the object the asynchronous function was
+ * started for and any error codes returned. The asynchronous callback
+ * function is then expected to call the corresponding "_finish()"
+ * function, passing the object the function was called for, the
+ * #GAsyncResult instance, and (optionally) an @error to grab any
+ * error conditions that may have occurred.
  *
  * The "_finish()" function for an operation takes the generic result
  * (of type #GAsyncResult) and returns the specific result that the