Update DerivedShape docstring (#536)

A comment is added to make it clear that `DerivedShape`s cannot drive thermal expansion
terrapower · Jan 13, 2022 · 21a451a · 21a451a
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commit 21a451a
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diff --git a/armi/reactor/components/__init__.py b/armi/reactor/components/__init__.py
@@ -286,6 +286,11 @@ class DerivedShape(UnshapedComponent):
     ----
     - This component type is "derived" through the addition or
       subtraction of other shaped components (e.g. Coolant)
+    - Because its area and volume are defined by other components,
+      a DerivedShape's area and volume may change as the other
+      components thermally expand. However the DerivedShape cannot
+      drive thermal expansion itself, even if it is a solid component
+      with non-zero thermal expansion coefficient
     """
 
     def getBoundingCircleOuterDiameter(self, Tc=None, cold=False):