docs: link Interface from others

Author: MilesCranmer

src/Expression.jl

@@ -42,7 +42,8 @@ end
 both the raw expression tree operating on a value type of `T`,
 as well as associated metadata to evaluate and render the expression.
 
-See `ExpressionInterface` for a full description of the interface implementation.
+See [`ExpressionInterface`](@ref) for a full description of the interface implementation,
+as well as tests to verify correctness.
 
 If you wish to use `@parse_expression`, you can also
 customize the parsing behavior with

src/Node.jl

@@ -45,6 +45,9 @@ this additionally must have fields for:
 
 # Interface
 
+See [`NodeInterface`](@ref) for a full description of the interface implementation,
+as well as tests to verify correctness.
+
 You *must* define `CustomNode{_T} where {_T} = new{_T}()` for each custom node type.
 
 In addition, you *may* choose to define the following functions, to override