bpo-36797: Prune more legacy distutils documentation (GH-13092)

Removes more legacy distutils documentation, and more clearly
marks what is left as potentially outdated, with references to
setuptools as a replacement.
diff --git a/Doc/distutils/commandref.rst b/Doc/distutils/commandref.rst
index 6a2ac96..0f6fe2a 100644
--- a/Doc/distutils/commandref.rst
+++ b/Doc/distutils/commandref.rst
@@ -4,6 +4,8 @@
 Command Reference
 *****************
 
+.. include:: ./_setuptools_disclaimer.rst
+
 .. % \section{Building modules: the \protect\command{build} command family}
 .. % \label{build-cmds}
 .. % \subsubsection{\protect\command{build}}