Simplify WL systematics description of dependence on star/gal size ratio

(LSSTScienceCollaborations#630)

whitepaper/Cosmology/wl.tex

@@ -50,13 +50,12 @@ \section{Weak Lensing}
 
 where $m_i$ is the multiplicative and $c_i$ is the additive systematic in the
 ith shear component.  These systematics have contributions from the atmosphere
-and the detector+optics.  Systematic errors in the PSF of the relatively
-brighter calibration stars in the field are propagated to errors in the galaxy
-shear, depending on the relative half-light sizes of the  galaxy and star.  To
-leading order, the PSF contributions to the additive systematic are a linear
-function of the PSF ellipticity.  The best observing strategies cause the
-average PSF ellipticity at a given point (over all exposures) to average towards
-zero.
+and the detector+optics.  Systematic errors in modeling the PSF from images of
+stars and in interpolating the PSF from the positions of stars to the positions of
+galaxies propagate to systematic errors in the galaxy shear.  To leading order, the
+PSF contributions to the additive systematic are a linear function of the PSF
+ellipticity.  The best observing strategies cause the average PSF ellipticity at a
+given point (over all exposures) to average towards zero.
 
 From the LSST SRD requirements on residual systematics in the galaxy shear-shear
 correlation function one can specify the level of residual shear systematics at