Update documentation to reflect new expectation.

Author: jaraco

importlib_metadata/docs/index.rst

@@ -12,8 +12,9 @@ efficient ``pkg_resources`` package.
 
 ``importlib_metadata`` is a backport of Python 3.8's standard library
 `importlib.metadata`_ module for Python 2.7, and 3.4 through 3.7.  Users of
-Python 3.8 and beyond are encouraged to use the standard library module, and
-in fact for these versions, ``importlib_metadata`` just shadows that module.
+Python 3.8 and beyond are encouraged to use the standard library module.
+When imported on Python 3.8 and later, ``importlib_metadata`` replaces the
+DistributionFinder behavior from the stdlib, but leaves the API in tact.
 Developers looking for detailed API descriptions should refer to the Python
 3.8 standard library documentation.