Document that using overrides disables all wheels

See pypa#4118. Update documentation to state that using command line options
on any requirements will disable all usage of wheels. Since this is
unexpected behavior, move text to a warning.

Author: martey

docs/html/reference/pip_install.rst

@@ -315,8 +315,12 @@ Per-requirement Overrides
 -------------------------
 
 Since version 7.0 pip supports controlling the command line options given to
-``setup.py`` via requirements files. This disables the use of wheels (cached or
-otherwise) for that package, as ``setup.py`` does not exist for wheels.
+``setup.py`` via requirements files.
+
+.. warning::
+
+This disables the use of wheels (cached or otherwise),
+as ``setup.py`` does not exist for wheels.
 
 The ``--global-option`` and ``--install-option`` options are used to pass
 options to ``setup.py``. For example: