Update general docstring

src/gradients/gradients.jl

 """
 Module defining sub-types of the [`GradientBlock`](@ref), i.e., any [`BuildingBlock`](@ref) that only defines a gradient profile.
 
+In principle, there are only three types of [`GradientBlock`]:
+- [`ChangingGradientBlock`](@ref): any gradient changing linearly in strength.
+- [`ConstantGradientBlock`](@ref): any gradient staying constant in strength. These can overlap with a pulse (`SliceSelectPulse`).
+- [`InstantGradientBlock`](@ref): an infinitely short gradient pulse.
+
+All other gradient profiles are made up of [`ChangingGradientBlock`](@ref) and [`ConstantGradientBlock`](@ref) and hence are technically sequences.
+They are still included here for clarity:
 - [`PulsedGradient`](@ref)
-- [`InstantGradientBlock`](@ref)
+- [`GradientWaveform`](@ref)
 - [`FixedGradient`](@ref)
 """
 module Gradients
 
+include("constant_gradient_blocks.jl")
+include("changing_gradient_blocks.jl")
+include("slice_selects.jl")
+
 #include("integrate_gradients.jl")
 include("fixed_gradients.jl")
 include("pulsed_gradients.jl")