clarify units for U1 and Uv temperature model coeff

* closes #945 
* group denominator together, same way it was done for U0 and Uc, ie: `W/(m^2 C)` instead of `(W/m^2 C)` which is ambiguous because it could mean `W/m^2 * C = W * C / m^2` which is not the same thing
* add missing division symbol before 2nd denominator `(m/s)` to get `W / (m^2 C) / (m/s)` instead of `(W/m^2 C) (m/s)` which looks like `WmC/ (m^2 s)` so it's ambiguous
* pvsyst help docs (https://www.pvsyst.com/help/thermal_loss.htm) say  `W/m²·k / m/s` which is also ambiguous but you can't win em all

Author: mikofski

pvlib/temperature.py

@@ -293,7 +293,7 @@ def pvsyst_cell(poa_global, temp_air, wind_speed=1.0, u_c=29.0, u_v=0.0,
 
     u_v : float, default 0.0
         Combined heat loss factor influenced by wind. Parameter :math:`U_{v}`
-        in :eq:`pvsyst` [(W/m^2 C)(m/s)].
+        in :eq:`pvsyst` [W/(m^2 C)/(m/s)].
 
     eta_m : numeric, default 0.1
         Module external efficiency as a fraction, i.e., DC power / poa_global.
@@ -383,7 +383,7 @@ def faiman(poa_global, temp_air, wind_speed=1.0, u0=25.0, u1=6.84):
 
     u1 : numeric, default 6.84
         Combined heat loss factor influenced by wind. The default value is one
-        determined by Faiman for 7 silicon modules. [(W/m^2 C)(m/s)].
+        determined by Faiman for 7 silicon modules. [W/(m^2 C)/(m/s)].
 
     Returns
     -------