Clarify distutils’ clean command (ref #6142)

Doc/distutils/apiref.rst

@@ -1934,8 +1934,12 @@ Subclasses of :class:`Command` must define the following methods.
 .. module:: distutils.command.clean
    :synopsis: Clean a package build area
 
+This command removes the temporary files created by :command:`build`
+and its subcommands, like intermediary compiled object files.  With
+the ``--all`` option, the complete build directory will be removed.
 
-.. % todo
+Extension modules built :ref:`in place <distutils-build-ext-inplace>`
+will not be cleaned, as they are not in the build directory.
 
 
 :mod:`distutils.command.config` --- Perform package configuration

Doc/distutils/configfile.rst

@@ -69,6 +69,8 @@ universal :option:`--help` option, e.g. ::
 Note that an option spelled :option:`--foo-bar` on the command-line  is spelled
 :option:`foo_bar` in configuration files.
 
+.. _distutils-build-ext-inplace:
+
 For example, say you want your extensions to be built "in-place"---that is, you
 have an extension :mod:`pkg.ext`, and you want the compiled extension file
 (:file:`ext.so` on Unix, say) to be put in the same source directory as your