Clarify docs on where .pth files can exist.

Doc/lib/libsite.tex

@@ -26,7 +26,8 @@ path for configuration files.
 \indexii{site-packages}{directory}
 
 A path configuration file is a file whose name has the form
-\file{\var{package}.pth}; its contents are additional items (one
+\file{\var{package}.pth} and exists in one of the four directories
+mentioned above; its contents are additional items (one
 per line) to be added to \code{sys.path}.  Non-existing items are
 never added to \code{sys.path}, but no check is made that the item
 refers to a directory (rather than a file).  No item is added to