Update UPGRADING for proc_get_status() and proc_close() change

Author: nielsdos

UPGRADING

@@ -174,6 +174,10 @@ PHP 8.1 UPGRADE NOTES
     reference representation since PHP 7.0.
   . debug_zval_dump() will now print "interned" instead of a dummy refcount of
     one for interned strings and immutable arrays.
+  . Executing proc_get_status() multiple times will now always return the right
+    value. Previously, only the first call of the function returned the right
+    value. Executing proc_close() after proc_get_status() will now also return
+    the right exit code. Previously this would return -1.
 
 - SPL:
   . SplFixedArray will now be JSON encoded like an array.