update solar_position docstring (#327)

* update solar_position docstring

* change units for pressure to mbars

* add note to whatsnew-0.4.5 about mbars in low-level spa.py module ONLY

Author: mikofski

docs/sphinx/source/whatsnew/v0.4.5.txt

@@ -13,6 +13,11 @@ Bug fixes
 * Added lower accuracy formulas for equation of time, declination, hour angle
   and solar zenith.
 * Remove all instances of .ix (:issue:`322`)
+* Update docstring in `pvlib.spa.solar_position` - change units of pressure to
+  millibars. *NOTE*: units of pressure in `pvlib.solar_position.spa_python` and
+  `pvlib.solar_position.spa_c` are still Pascals. This update should have no
+  effect on most users, since it only applies to the low-level `spa.py` module.
+  (:issue:`327`)
 
 
 Enhancements

pvlib/spa.py

@@ -1103,7 +1103,7 @@ def solar_position(unixtime, lat, lon, elev, pressure, temp, delta_t,
     elev : float
         Elevation of location in meters
     pressure : int or float
-        avg. yearly pressure at location in Pascals;
+        avg. yearly pressure at location in millibars;
         used for atmospheric correction
     temp : int or float
         avg. yearly temperature at location in