update docs for vips_shutdown() use

Since we have a new policy.

ChangeLog

@@ -8,6 +8,7 @@
   memory and file descriptors
 - add vips_image_get_format_max()
 - flatten handles out of range alpha and max_alpha correctly
+- don't use atexit for cleanup, it's too unreliable
 
 16/8/21 started 8.11.4
 - fix off-by-one error in new rank fast path

doc/using-C.xml

@@ -29,8 +29,13 @@
     <para>
       When your program starts, use VIPS_INIT()
       to start up the VIPS library. You should pass it the name
-      of your program, usually <code>argv[0]</code>. Use
-      vips_shutdown() when you exit. VIPS_INIT() is a macro to let it check 
+      of your program, usually <code>argv[0]</code>. If you call
+      vips_shutdown() just before you exit, libvips will attempt to free all 
+      resources. This can help leak checking, but is not required.
+    </para>
+
+    <para>
+      VIPS_INIT() is a macro to let it check 
       that the libvips library you have linked to matches the libvips headers 
       you included. 
     </para>

libvips/iofuncs/init.c

@@ -30,6 +30,10 @@
  * 	- hide warnings is VIPS_WARNING is set
  * 20/4/19
  * 	- set the min stack, if we can
+ * 17/9/21
+ * 	- don't use atexit for cleanup, it's too unreliable ... users should
+ * 	  call vips_shutdown explicitly if they want a clean exit, though a
+ * 	  dirty exit is fine
  */
 
 /*
@@ -726,6 +730,9 @@ vips_thread_shutdown( void )
  * Call this to drop caches and close plugins. Run with "--vips-leak" to do 
  * a leak check too. 
  *
+ * vips_shutdown() is optional. If you don't call it, your platform will
+ * clean up for you.
+ *
  * You may call VIPS_INIT() many times and vips_shutdown() many times, but you 
  * must not call VIPS_INIT() after vips_shutdown(). In other words, you cannot
  * stop and restart vips. 
@@ -754,11 +761,8 @@ vips_shutdown( void )
 }
 
 	vips__render_shutdown();
-
 	vips_thread_shutdown();
-
 	vips__thread_profile_stop();
-
 	vips__threadpool_shutdown();
 
 #ifdef HAVE_GSF