gh-115075: Fix info about initialization of mimetypes.MimeTypes attributes (GH-150691)

They are no longer depended on the current value of global variables since
Python 3.9 (see gh-49213).

Doc/library/mimetypes.rst

diff --git a/Doc/library/mimetypes.rst b/Doc/library/mimetypes.rst
index f6cfbb5..bd24079 100644
--- a/Doc/library/mimetypes.rst
+++ b/Doc/library/mimetypes.rst
@@ -211,8 +211,8 @@
 .. class:: MimeTypes(filenames=(), strict=True)
 
    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.
@@ -226,30 +226,32 @@
       Dictionary mapping suffixes to suffixes.  This is used to allow recognition of
       encoded files for which the encoding and the type are indicated by the same
       extension.  For example, the :file:`.tgz` extension is mapped to :file:`.tar.gz`
-      to allow the encoding and type to be recognized separately.  This is initially a
-      copy of the global :data:`suffix_map` defined in the module.
+      to allow the encoding and type to be recognized separately.
+      This is initialized with some predefined values.
 
 
    .. attribute:: MimeTypes.encodings_map
 
-      Dictionary mapping filename extensions to encoding types.  This is initially a
-      copy of the global :data:`encodings_map` defined in the module.
+      Dictionary mapping filename extensions to encoding types.
+      This is initialized with some predefined values.
 
 
    .. attribute:: MimeTypes.types_map
 
       Tuple containing two dictionaries, mapping filename extensions to MIME types:
       the first dictionary is for the non-standards types and the second one is for
-      the standard types. They are initialized by :data:`common_types` and
-      :data:`types_map`.
+      the standard types.
+      They are initialized with some predefined values and MIME type
+      information loaded from files specified by the *filenames* argument.
 
 
    .. attribute:: MimeTypes.types_map_inv
 
       Tuple containing two dictionaries, mapping MIME types to a list of filename
       extensions: the first dictionary is for the non-standards types and the
-      second one is for the standard types. They are initialized by
-      :data:`common_types` and :data:`types_map`.
+      second one is for the standard types.
+      They are initialized with some predefined values and MIME type
+      information loaded from files specified by the *filenames* argument.
 
 
    .. method:: MimeTypes.guess_extension(type, strict=True)