Stackless issue #248: Documentation update

Mention, that module "stackless" must be imported, to enable the
extended pickling capabilities.

(cherry picked from commit c5cabba)

Author: Anselm Kruis

Doc/library/stackless/pickling.rst

@@ -132,7 +132,8 @@ Pickling other objects
 ======================
 
 In order to be able to pickle tasklets |SLP| needs to be able to pickle
-several other objects, which can't be pickled by |CPY|. |SLP|
+several other objects, which can't be pickled by |CPY|. If the module
+:mod:`stackless` gets imported for the first time, |SLP|
 uses :func:`copyreg.pickle` to register “reduction” functions for the following
 types:
 :data:`~types.AsyncGeneratorType`,

Stackless/changelog.txt

@@ -9,6 +9,10 @@ What's New in Stackless 3.X.X?
 
 *Release date: 20XX-XX-XX*
 
+- https://github.com/stackless-dev/stackless/issues/248
+  Documentation update: Mention, that module "stackless" must be imported, to
+  enable the extended pickling capabilities.
+
 - https://github.com/stackless-dev/stackless/issues/239
   Add support for PEP 567 context variables to tasklets.
   Each tasklet now has a contextvars.Context object, that becomes active during