[glib/mcatanzaro/g-assert-error-doc] Improve documentation of g_assert_error()

[Michael Catanzaro <mcatanzaro src gnome org>]

commit 0618d67411a6144de2263eea76df7ec53b402edd
Author: Michael Catanzaro <mcatanzaro posteo net>
Date:   Wed Dec 19 02:29:14 2018 +0000

    Improve documentation of g_assert_error()
    
    This macro is intended for use in tests, but recommends as an
    alternative to use a macro that is not intended for use in tests.
    Fix it.

 glib/gtestutils.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/glib/gtestutils.c b/glib/gtestutils.c
index 9252ebbf5..4b008f34d 100644
--- a/glib/gtestutils.c
+++ b/glib/gtestutils.c
@@ -751,7 +751,7 @@
  *
  * This can only be used to test for a specific error. If you want to
  * test that @err is set, but don't care what it's set to, just use
- * `g_assert (err != NULL)`
+ * `g_assert_nonnull (err)`.
  *
  * Since: 2.20
  */