backport async_fallback deprecation changelog

patched from 56f7b5391a34eb013fee7150a7206

Change-Id: Ida36037c629a8deed2a13e593132fcae9ceb8da0

diff --git a/doc/build/changelog/unreleased_20/async_fallback.rst b/doc/build/changelog/unreleased_20/async_fallback.rst
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+.. change::
+    :tags: change, asyncio
+
+    The ``async_fallback`` dialect argument is now deprecated, and will be
+    removed in SQLAlchemy 2.1.   This flag has not been used for SQLAlchemy's
+    test suite for some time.   asyncio dialects can still run in a synchronous
+    style by running code within a greenlet using :func:`_util.greenlet_spawn`.