Update pyenv docs to avoid using shim

Also removed the PIPENV_PYTHON suggestion, as this is likely to cause
confusion if added to e.g. .bashrc - it will always be set to the
global pyenv.

Fixes pypa#4534

docs/diagnose.rst

@@ -58,14 +58,9 @@ distributions, with version name like ``3.6.4`` or similar.
 ------------------------------------------------------------------
 
 Pipenv by default uses the Python it is installed against to create the
-virtualenv. You can set the ``--python`` option, or
-``$PYENV_ROOT/shims/python`` to let it consult pyenv when choosing the
-interpreter. See :ref:`specifying_versions` for more information.
-
-If you want Pipenv to automatically “do the right thing”, you can set the
-environment variable ``PIPENV_PYTHON`` to ``$PYENV_ROOT/shims/python``. This
-will make Pipenv use pyenv’s active Python version to create virtual
-environments by default.
+virtualenv. You can set the ``--python`` option to ``$(pyenv which python)``
+to use your current pyenv interpreter. See :ref:`specifying_versions` for more
+information.
 
 .. _unknown-local-diagnose:
 

news/4534.doc.rst

@@ -0,0 +1 @@
+Fix suggested pyenv setup to avoid using shimmed interpreter