Refactor TJOperators to the dense-first strategy

docs/src/tjoperators.md

@@ -1,9 +1,19 @@
 ```@meta
+CurrentModule = TensorKitTensors.TJOperators
 CollapsedDocStrings = true
 ```
 
 # TJ operators
 
+## Relation to the Hubbard model
+
+The t-J model is the Hubbard model restricted to the subspace without double occupancy. The
+operators of this module are defined accordingly: each of them is the projection of the
+[Hubbard operator](hubbardoperators.md) of the same name onto the t-J space, through the
+``3 ← 4`` isometry [`tj_projector`](@ref). The only Hubbard operators without a t-J
+counterpart are the double-occupancy operators `ud_num` and `half_ud_num`, which project to
+zero.
+
 ```@autodocs
-Modules = [TensorKitTensors.TJOperators]
+Modules = [TJOperators]
 ```