bpo-40611: Adds MAP_POPULATE to the mmap module (GH-20061)

MAP_POPULATE constant has now been added to the list of exported
mmap module flags. 
diff --git a/Doc/library/mmap.rst b/Doc/library/mmap.rst
index 1f3fbc3..698c176 100644
--- a/Doc/library/mmap.rst
+++ b/Doc/library/mmap.rst
@@ -81,7 +81,9 @@
    private copy-on-write mapping, so changes to the contents of the mmap
    object will be private to this process, and :const:`MAP_SHARED` creates a
    mapping that's shared with all other processes mapping the same areas of
-   the file.  The default value is :const:`MAP_SHARED`.
+   the file.  The default value is :const:`MAP_SHARED`. Some systems have
+   additional possible flags with the full list specified in
+   :ref:`MAP_* constants <map-constants>`.
 
    *prot*, if specified, gives the desired memory protection; the two most
    useful values are :const:`PROT_READ` and :const:`PROT_WRITE`, to specify
@@ -342,3 +344,21 @@
    Availability: Systems with the madvise() system call.
 
    .. versionadded:: 3.8
+
+.. _map-constants:
+
+MAP_* Constants
++++++++++++++++
+
+.. data:: MAP_SHARED
+          MAP_PRIVATE
+          MAP_DENYWRITE
+          MAP_EXECUTABLE
+          MAP_ANON
+          MAP_ANONYMOUS
+          MAP_POPULATE
+
+    These are the various flags that can be passed to :meth:`mmap.mmap`. Note that some options might not be present on some systems.
+
+    .. versionchanged:: 3.10
+       Added MAP_POPULATE constant.