Document adjoint plans

Author: gaurav-arya

src/definitions.jl

@@ -600,6 +600,14 @@ mutable struct AdjointPlan{T,P<:Plan} <: Plan{T}
     AdjointPlan{T,P}(p) where {T,P} = new(p)
 end
 
+"""
+    Base.adjoint(p::Plan)
+
+Form the adjoint operator of an FFT plan. Returns a plan `p'` which performs the adjoint operation
+the original plan. Note that this differs from the corresponding backwards plan in the case of real
+FFTs due to the halving of one of the dimensions of the FFT output, as described in [`rfft`](@ref).
+Adjoint plans do not currently support `mul!`.
+"""
 Base.adjoint(p::Plan{T}) where {T} = AdjointPlan{T, typeof(p)}(p)
 Base.adjoint(p::AdjointPlan) = p.p
 # always have AdjointPlan inside ScaledPlan.