Clarify action and requirements of recenter

Author: timholy

src/core.jl

@@ -74,8 +74,17 @@ Base.inv(trans::IdentityTransformation) = trans
     recenter(trans::Union{AbstractMatrix,Transformation}, origin::AbstractVector) -> ctrans
 
 Return a new transformation `ctrans` such that point `origin` serves
-as the origin-of-coordinates for `trans`. For example, if `trans` is a
-rotation matrix, then `ctrans` is a rotation around `origin`.
+as the origin-of-coordinates for `trans`. Translation by `±origin`
+occurs both before and after applying `trans`, so that if `trans` is
+linear we have
+
+    ctrans(origin) == origin
+
+As a consequence, `recenter` only makes sense if the output space of
+`trans` is isomorphic with the input space.
+
+For example, if `trans` is a rotation matrix, then `ctrans` rotates
+space around `origin`.
 """
 function recenter(trans::Transformation, origin::AbstractVector)
     Translation(origin) ∘ trans ∘ Translation(-origin)