Document the OS-Unsupported label and Platforms experts list (GH-1281)

Co-Authored-By: Erlend E. Aasland <erlend@python.org>

core-developers/experts.rst

@@ -282,14 +282,21 @@ PEG Generator       gvanrossum, pablogsal, lysnikolaou
 ==================  ===========
 
 
+.. _platform-experts:
+
 Platforms
 =========
 
+For official contacts for supported platforms, see :pep:`11`.
+
+Platforms listed here are not necessarily supported by CPython.
+Some of the experts listed here maintain and distribute Python
+for “their” platform as a third-party project.
+
 ===================   ===========
 Platform              Maintainers
 ===================   ===========
 AIX                   David.Edelsohn^
-Android
 Cygwin                jlt63^, stutzbach^
 Emscripten            hoodmane, pmp-p, rdb, rth, ryanking13
 FreeBSD

triage/labels.rst

@@ -61,9 +61,14 @@ OS labels
 
 These labels are used to specify which operating systems are affected.
 Since most issues either affect all systems or are specific to Unix,
-the only available labels are :gh-label:`OS-windows`, :gh-label:`OS-mac`,
-and :gh-label:`OS-freebsd`.
-
+we don't have a dedicated Unix label.
+Use :gh-label:`OS-windows`, :gh-label:`OS-mac`, and :gh-label:`OS-freebsd`.
+
+Use the :gh-label:`OS-unsupported` label for issues on platforms outside the
+support tiers defined in :pep:`11`. Applying this label adds the issue to
+`a GitHub project <https://github.com/orgs/python/projects/27/views/1>`_ where
+it can be categorized further.
+See also the :ref:`Platform experts list <platform-experts>`.
 
 .. _Expert labels:
 .. _Topic labels: