bpo-46826: add documentation for prefixes argument on site.getsitepackages

Author: asnelling

Doc/library/site.rst

@@ -211,12 +211,19 @@ Module contents
    used in :mod:`sitecustomize` or :mod:`usercustomize` (see above).
 
 
-.. function:: getsitepackages()
+.. function:: getsitepackages(prefixes=None)
 
-   Return a list containing all global site-packages directories.
+   Return a list containing all global :file:`site-packages` directories.
+
+   For each directory present in *prefixes* (or :data:`PREFIXES` if *prefixes*
+   is None), this function will find its :file:`site-packages` subdirectory
+   depending on the system environment, and will return a list of full paths.
 
    .. versionadded:: 3.2
 
+   .. versionchanged:: 3.3
+      Add the optional *prefixes* argument.
+
 
 .. function:: getuserbase()