gh-90982: Document prefixes argument on site.getsitepackages (GH-31546)

(cherry picked from commit bc31217)

Co-authored-by: Addison Snelling <addison@asnell.io>

Author: asnelling

Doc/library/site.rst

@@ -233,12 +233,19 @@ 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.
 
+   For each directory present in *prefixes* (or :data:`PREFIXES` if *prefixes*
+   is ``None``), this function will find its 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()