Docs: Add deprecation of Image.__del__ (#3929)

Docs: Add deprecation of Image.__del__

docs/deprecations.rst

@@ -12,6 +12,29 @@ Deprecated features
 Below are features which are considered deprecated. Where appropriate,
 a ``DeprecationWarning`` is issued.
 
+Image.__del__
+~~~~~~~~~~~~~
+
+.. deprecated:: 6.1.0
+
+Implicitly closing the image's underlying file in ``Image.__del__`` has been deprecated.
+Use a context manager or call ``Image.close()`` instead to close the file in a
+deterministic way.
+
+Deprecated:
+
+.. code-block:: python
+
+    im = Image.open("hopper.png")
+    im.save("out.jpg")
+
+Use instead:
+
+.. code-block:: python
+
+    with Image.open("hopper.png") as im:
+        im.save("out.jpg")
+
 Python 2.7
 ~~~~~~~~~~
 

docs/releasenotes/6.1.0.rst

@@ -1,6 +1,32 @@
 6.1.0
 -----
 
+Deprecations
+============
+
+Image.__del__
+^^^^^^^^^^^^^
+
+.. deprecated:: 6.1.0
+
+Implicitly closing the image's underlying file in ``Image.__del__`` has been deprecated.
+Use a context manager or call ``Image.close()`` instead to close the file in a
+deterministic way.
+
+Deprecated:
+
+.. code-block:: python
+
+    im = Image.open("hopper.png")
+    im.save("out.jpg")
+
+Use instead:
+
+.. code-block:: python
+
+    with Image.open("hopper.png") as im:
+        im.save("out.jpg")
+
 API Additions
 =============