Update Doc/library/mimetypes.rst

Co-authored-by: Stan Ulbrych <stan@python.org>

Author: serhiy-storchaka

Doc/library/mimetypes.rst

@@ -209,7 +209,8 @@ than one MIME-type database; it provides an interface similar to the one of the
 
    This class represents a MIME-types database.  By default, it provides access to
    the same database as the rest of this module. The initial database is a copy of
-   that provided by the module, and may be extended by loading additional
+the same database as the rest of this module. The initial database is created from
+Python's built-in MIME type tables. It may be extended by loading additional
    :file:`mime.types`\ -style files into the database using the :meth:`read` or
    :meth:`readfp` methods.  The mapping dictionaries may also be cleared before
    loading additional data if the default data is not desired.