Resolve ambiguity regarding detector orientation

scico/linop/xray/__init__.py

@@ -26,7 +26,14 @@
    :align: center
    :include-source: False
    :show-source-link: False
-   :caption: Comparison of 2D X-ray projector geometries
+   :caption: Comparison of 2D X-ray projector geometries. The red arrows
+      are are directed towards the detector, which is oriented with pixel
+      indices ordered in the same direction as clockwise rotation (e.g.
+      in the "scico" geometry, the :math:`\theta=0` projection
+      corresponds to row sums ordered from the top to the bottom of the
+      figure, while the :math:`\theta=\pi` projection
+      corresponds to row sums ordered from the bottom to the top of the
+      figure).
 
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