bpo-38623: Doc: Add section for site module CLI. (GH-17858)

(cherry picked from commit ca94677) Co-authored-by: Inada Naoki <songofacandy@gmail.com>

Author: miss-islington

Doc/library/site.rst

@@ -223,6 +223,13 @@ Module contents
  .. versionadded:: 3.2
 
 
+.. _site-commandline:
+
+Command Line Interface
+----------------------
+
+.. program:: site
+
 The :mod:`site` module also provides a way to get the user directories from the
 command line:
 
@@ -231,8 +238,6 @@ command line:
  $ python3 -m site --user-site
  /home/user/.local/lib/python3.3/site-packages
 
-.. program:: site
-
 If it is called without arguments, it will print the contents of
 :data:`sys.path` on the standard output, followed by the value of
 :data:`USER_BASE` and whether the directory exists, then the same thing for