Add a note about edge sources

Author: melanieclarke

docs/jwst/adaptive_trace_model/main.rst

@@ -93,6 +93,10 @@ Slope values higher than a threshold value (step parameter ``slope_limit``) indi
 a compact source region.  The trace model will be evaluated for these regions, with some
 padding for nearby pixels; it will not be evaluated in other regions.
 
+For MOS and fixed slit data, note that the model accuracy also depends on the position of the
+source within the spectral region.  Sources on the edge of a MOS slitlet, for example,
+may not be well modeled by the spline fits, even if they are bright and compact.
+
 If no oversampling is desired (i.e., the ``oversample`` parameter is set to 1.0), then the trace
 model is evaluated at every input pixel in a compact source region to create a wavelength-dependent
 spatial profile.  This image is stored in the output datamodel, in the ``trace_model`` attribute.