Add a porting guide entry for ansible_distribution facts (#50251)

* Add a porting guide entry for ansible_distribution facts

Switching away from platform.distro() will cause changes sometimes due
to the new code using new sources of information that may be out of sync
with the old ones.  Just have to make people aware of that and also what
we are doing to mitigate it when appropriate.

* wordsmithed, added links for new distro backend

changelogs/fragments/platform-dist-to-nir0s-distro.yaml

@@ -1,5 +1,7 @@
 ---
 minor_changes:
 - Python-3.8 removes platform.dist() from the standard library. To maintain
-  compatibility we've switched to an alternative library, nir0s/distro to
-  detect the distribution for fact gathering.
+  compatibility we've switched to an alternative library, nir0s/distro, to
+  detect the distribution for fact gathering.  Distributions facts may change
+  slightly as nir0s/distro has bugfixes which the standard library's
+  platform.dist() has lacked.

docs/docsite/rst/porting_guides/porting_guide_2.8.rst

@@ -17,7 +17,15 @@ This document is part of a collection on porting. The complete list of porting g
 Playbook
 ========
 
-No notable changes.
+Distribution Facts
+------------------
+
+The information returned for the ``ansible_distribution_*`` group of facts may have changed
+slightly.  Ansible 2.8 uses a new backend library for information about distributions: `nir0s/distro <https://github.com/nir0s/distro>`_. This library runs on Python-3.8 and fixes many bugs, including correcting release and version names.
+
+The two facts used in playbooks most often, ``ansible_distribution`` and ``ansible_distribution_major_version``, should not change. If you discover a change in these facts, please file a bug so we can address the
+difference.  However, other facts like ``ansible_distribution_release`` and
+``ansible_distribution_version`` may change as erroneous information gets corrected.
 
 
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