add missing area parameter for eta_m arg (#1182)

* add missing area parameter for eta_m arg

* Remove trailing whitespace

* Add LaTeX for formatting

* Fix line too long

* Remove trailing whitespace

* Correct formula

Co-authored-by: Cliff Hansen <[email protected]>

* Improve presentation of the formula

Co-authored-by: Cliff Hansen <[email protected]>

Author: KushalBeniwal

pvlib/temperature.py

@@ -316,7 +316,8 @@ def pvsyst_cell(poa_global, temp_air, wind_speed=1.0, u_c=29.0, u_v=0.0,
         :math:`\left[ \frac{\text{W}/\text{m}^2}{\text{C}\ \left( \text{m/s} \right)} \right]`
 
     eta_m : numeric, default 0.1
-        Module external efficiency as a fraction, i.e., DC power / poa_global.
+        Module external efficiency as a fraction, i.e.,
+        :math:`DC\ power / (POA\ irradiance \times module\ area)`.
         Parameter :math:`\eta_{m}` in :eq:`pvsyst`.
 
     alpha_absorption : numeric, default 0.9