Slight doc improvements

See #320

Author: waynew

aiosmtpd/docs/concepts.rst

@@ -2,6 +2,8 @@
  Concepts
 ==========
 
+This guide assumes familiarity with the SMTP protocol.
+
 There are two general ways you can run the SMTP server, via the
 :ref:`command line <cli>` or :ref:`programmatically <controller>`.
 

aiosmtpd/docs/intro.rst

@@ -9,7 +9,11 @@ Simple Mail Transfer Protocol (SMTP) and
 Local Mail Transfer Protocol (LMTP).  It is derived from
 `Python 3.5's smtpd.py <https://hg.python.org/cpython/file/3.5/Lib/smtpd.py>`__
 standard library module, and provides both a command line interface and an API
-for use in testing applications that send email.
+for use in testing applications that send email. Since the `smtpd module was
+removed in Python 3.12
+<https://docs.python.org/3.12/whatsnew/3.12.html#locale>`, following `PEP 594
+<https://peps.python.org/pep-0594/#smtpd>`, aiosmtpd is the recommended
+replacement.
 
 Inspiration for this library comes from several other packages: