JAL-3111 tweak PCA documentation

diff --git a/help/html/calculations/pca.html b/help/html/calculations/pca.html
index 5b76d10..3529cae 100755 (executable)
--- a/help/html/calculations/pca.html
+++ b/help/html/calculations/pca.html
@@ -78,8 +78,10 @@
     within the similarity space, as points in a rotateable 3D
     scatterplot. The colour of each sequence point is the same as the
     sequence group colours, white if no colour has been defined for the
-    sequence, and green if the sequence is part of a the currently
-    selected group.</p>
+    sequence, and grey if the sequence is part of the currently selected
+    group. The viewer also employs depth cueing, so points appear darker
+    the farther away they are, and become brighter as they are rotated
+    towards the front of the view.</p>
   <p>
     The 3d view can be rotated by dragging the mouse with the <strong>left
       mouse button</strong> pressed. The view can also be zoomed in and out with