gh-143387: Update docs to reflect the behavior and note the changed version.

Author: jaraco

Doc/library/importlib.metadata.rst

@@ -224,6 +224,9 @@ Distribution metadata
    Raises :exc:`PackageNotFoundError` if the named distribution
    package is not installed in the current Python environment.
 
+   Raises :exc:`MetadataNotFound` if a distribution package is
+   present but no METADATA file is present.
+
 .. class:: PackageMetadata
 
    A concrete implementation of the
@@ -252,6 +255,12 @@ all the metadata in a JSON-compatible form per :PEP:`566`::
 The full set of available metadata is not described here.
 See the PyPA `Core metadata specification <https://packaging.python.org/en/latest/specifications/core-metadata/#core-metadata>`_ for additional details.
 
+.. versionchanged:: 3.15
+   Previously and incidentally, if a METADATA file was missing from a distribution, an
+   empty ``PackageMetadata`` would be returned, indistinguishable from
+   an empty METADATA file. Now, a missing METADATA file triggers a
+   ``MetadataNotFound`` exception.
+
 .. versionchanged:: 3.10
    The ``Description`` is now included in the metadata when presented
    through the payload. Line continuation characters have been removed.
@@ -465,6 +474,9 @@ The same applies for :func:`entry_points` and :func:`files`.
    .. attribute:: metadata
       :type: PackageMetadata
 
+      Raises :exc:`MetadataNotFound` if the METADATA file is not present in
+      the distribution.
+
       There are all kinds of additional metadata available on :class:`!Distribution`
       instances as a :class:`PackageMetadata` instance::