DOC: clarify default depo_veloc values in soiling.hsu Notes (#2854)

* DOC: clarify default depo_veloc values in soiling.hsu Notes

* DOC: clarify default depo_veloc values in soiling.hsu Notes

---------

Co-authored-by: weixuan-song <weixuan-song@users.noreply.github.com>
Co-authored-by: Kevin Anderson <kevin.anderso@gmail.com>

Author: weixuan-song

docs/sphinx/source/whatsnew/v0.16.0.rst

@@ -75,6 +75,9 @@ Enhancements
 Documentation
 ~~~~~~~~~~~~~
 * Remove `g_poa_effective` from the :ref:`nomenclature` page (:pull:`2704`)
+* Clarify in the ``Notes`` section of :py:func:`pvlib.soiling.hsu` that the
+  default ``depo_veloc`` values correspond to the settling velocity of
+  particulates. (:issue:`2535`)
 * Correct the transposed table of Hellmann exponents in the
   :py:func:`pvlib.atmosphere.windspeed_powerlaw` docstring, which disagreed
   with :py:data:`pvlib.atmosphere.HELLMANN_SURFACE_EXPONENTS` in four of its

pvlib/soiling.py

@@ -67,6 +67,10 @@ def hsu(rainfall, cleaning_threshold, surface_tilt, pm2_5, pm10,
     corresponding to a soiling ratio of approximately 0.6875.
     See [1]_ for details.
 
+    As recommended by [1]_, the default values of ``depo_veloc``
+    are the settling velocities of particulates, i.e., only the gravitational
+    movement of particles.
+
     References
     -----------
     .. [1] M. Coello and L. Boyle, "Simple Model For Predicting Time Series