Add type headers to type docstrings

Author: abhro

src/core.jl

@@ -6,6 +6,8 @@
 # transform_deriv() and transform_deriv_params()
 
 """
+    abstract type Transformation
+
 The `Transformation` supertype defines a simple interface for performing
 transformations. Subtypes should be able to apply a coordinate system
 transformation on the correct data types by overloading the call method, and
@@ -15,6 +17,8 @@ Efficient compositions can optionally be defined by `compose()` (equivalently `
 abstract type Transformation end
 
 """
+    IdentityTransformation <: Transformation
+
 The `IdentityTransformation` is a singleton `Transformation` that returns the
 input unchanged, similar to `identity`.
 """
@@ -23,8 +27,10 @@ struct IdentityTransformation <: Transformation; end
 @inline (::IdentityTransformation)(x) = x
 
 """
-A `ComposedTransformation` simply executes two transformations successively, and
-is the fallback output type of `compose()`.
+    ComposedTransformation{T1, T2} <: Transformation
+
+A `ComposedTransformation` simply executes two transformations `T2` and `T1`
+successively, and is the fallback output type of `compose()`.
 """
 struct ComposedTransformation{T1 <: Transformation, T2 <: Transformation} <: Transformation
     t1::T1