Add hint about Perez discontinuities (#2098)

* Add hint about Perez discontinuities

* Update warning

* Apply suggestions from code review

Co-authored-by: Cliff Hansen <cwhanse@sandia.gov>

* Apply suggestions from code review

Co-authored-by: Echedey Luis <80125792+echedey-ls@users.noreply.github.com>

---------

Co-authored-by: Cliff Hansen <cwhanse@sandia.gov>
Co-authored-by: Echedey Luis <80125792+echedey-ls@users.noreply.github.com>

Author: 3 people

pvlib/irradiance.py

@@ -404,6 +404,11 @@ def get_sky_diffuse(surface_tilt, surface_azimuth,
     require ``'dni_extra'``. Values can be calculated using
     :py:func:`~pvlib.irradiance.get_extra_radiation`.
 
+    The ``'Perez'`` transposition model features discontinuities in the
+    predicted tilted diffuse irradiance due to relying on discrete input
+    values. For applications that benefit from continuous output, consider
+    using :py:func:`~pvlib.irradiance.perez_driesse`.
+
     The ``'perez'`` and ``'perez-driesse'`` models require relative airmass
     (``airmass``) as input. If ``airmass`` is not provided, it is calculated
     using the defaults in :py:func:`~pvlib.atmosphere.get_relative_airmass`.
@@ -1004,6 +1009,13 @@ def perez(surface_tilt, surface_azimuth, dhi, dni, dni_extra,
     pressure-corrected) airmass. Optionally a selector may be used to
     use any of Perez's model coefficient sets.
 
+    Warning
+    -------
+    The Perez transposition model features discontinuities in the
+    predicted tilted diffuse irradiance due to relying on discrete input
+    values. For applications that benefit from continuous output, consider
+    using :py:func:`~pvlib.irradiance.perez_driesse`.
+
     Parameters
     ----------
     surface_tilt : numeric