add a docstring for Rect and its aliases

src/primitives/rectangles.jl

@@ -14,6 +14,22 @@ end
 ##
 # Constructors & typealiases
 
+"""
+    const Rect{N,T} = HyperRectangle{N,T}
+
+A rectangle in N dimensions, formally the cartesian product of intervals. See also [`HyperRectangle`](@ref). Its aliases are
+
+|        |`T`(eltype)|`Float32` |`Int`     |
+|--------|-----------|----------|----------|
+|`N`(dim)|`Rect{N,T}`|`Rectf{N}`|`Recti{N}`|
+|`2`     |`Rect2{T}` |`Rect2f`  |`Rect2i`  |
+|`3`     |`Rect3{T}` |`Rect3f`  |`Rect3i`  |
+
+There is an additional unexported alias `RectT` that simply reverses the order of type parameters: `RectT{T,N} == Rect{N,T}`.
+
+"""
+Rect, Rect2, Rect3, RectT, Rectf, Rect2f, Rect3f, Recti, Rect2i, Rect3i
+
 const Rect{N,T} = HyperRectangle{N,T}
 const Rect2{T} = Rect{2,T}
 const Rect3{T} = Rect{3,T}