Tidy up writing

docs/src/developer_documentation/tangents.md

@@ -1,28 +1,28 @@
 # Tangents
 
-As discussed in [Representing Gradients](@ref), Mooncake requires that each "primal" type `P` be associated to a unique "tangent" type `T`, given by the function [tangent_type](@ref).
+As discussed in [Representing Gradients](@ref), Mooncake requires that each "primal" type be associated to a unique "tangent" type, given by the function [tangent_type](@ref).
 Moreover, we must be able to "split" a given tangent into its _fdata_ ("forwards-data") and _rdata_ ("reverse-data"), whose types are given by [fdata_type](@ref) and `rdata_type` respectively.
 
-Additionally, there is a range of things that one must be able to do with any given tangent / fdata / rdata for Mooncake to operate correctly.
-
-## Note
-
-The tangent type, and associated functionality, assigned automatically by Mooncake is completely adequate for the vast majority of primals.
-Before diving in to add your own custom type, seriously consider whether it is worth the effort.
+Very occassionally it may be necessary to specify your own tangent type.
+This is not an entirely trivial undertaking, as there is quite a lot of functionality that must be added to make it work properly.
+So, before diving in to add your own custom type, seriously consider whether it is worth the effort, and whether the default definition given by Mooncake are really inadequate for your use-case.
 
 ## Testing Functionality
 
-The functionality which must work on a given type is tested over three functions:
+The interface is given in the form of three functions:
 ```@docs
 Mooncake.TestUtils.test_tangent_interface
 Mooncake.TestUtils.test_tangent_splitting
 Mooncake.TestUtils.test_rule_and_type_interactions
 ```
+
 You can call all three of these functions at once using
 ```@docs
 Mooncake.TestUtils.test_data
 ```
 
+If all the tests in these functions pass, then you have satisfied the interface.
+
 ## Interface
 
 Below are the docstrings for each function tested by [`Mooncake.TestUtils.test_tangent_interface`](@ref) and [`Mooncake.TestUtils.test_tangent_splitting`](@ref).