gmain: Document that g_steal_fd() preserves errno

This is useful when writing similarly low-level code, and was always true
as implemented here; let's document it so that other codebases can rely
on it.

Signed-off-by: Simon McVittie <smcv@collabora.com>

glib/gmain.c

@@ -6608,6 +6608,8 @@ g_get_worker_context (void)
  * [`signal-safety(7)`](man:signal-safety(7))), making it safe to call from a
  * signal handler or a #GSpawnChildSetupFunc.
  *
+ * This function preserves the value of `errno`.
+ *
  * Returns: the value that @fd_ptr previously had
  * Since: 2.70
  */