Add docstring to _reinterpret_unsafe (#2745)

Co-authored-by: odow <o.dowson@gmail.com>

Author: blegat

src/Nonlinear/ReverseAD/utils.jl

@@ -164,9 +164,43 @@ function _UnsafeLowerTriangularMatrixView(x::Vector{Float64}, N::Int)
     return _UnsafeLowerTriangularMatrixView(N, pointer(x))
 end
 
+"""
+    _reinterpret_unsafe(::Type{T}, x::Vector{R}) where {T,R}
+
+Return an `_UnsafeVectorView` that act as a vector of element type
+`T` over the same bytes as `x`. Note that if `length(x) * sizeof(R)` is not
+a multiple of `sizeof(T)`, the last bits will be ignored. This is a key
+difference with `reinterpret` which errors in that case.
+
+Given a vector of `Float64` of length equal to the maximum number of nodes of a
+set of expressions time the maximum chunk size, this function is used to
+reinterpret it as a vector of `ForwardDiff.Partials{N,T}` where `N` is the
+chunk size of one of the expressions of the set. In that case, we know that
+the vector has enough bytes and we don't care about the leftover bytes at the
+end.
+
+## Examples
+
+```jldoctest
+julia> import MathOptInterface as MOI
+
+julia> x = [(1, 2, 3), (4, 5, 6), (7, 8, 9)]
+3-element Vector{Tuple{Int64, Int64, Int64}}:
+ (1, 2, 3)
+ (4, 5, 6)
+ (7, 8, 9)
+
+julia> MOI.Nonlinear.ReverseAD._reinterpret_unsafe(NTuple{2,Int}, x)
+4-element MathOptInterface.Nonlinear.ReverseAD._UnsafeVectorView{Tuple{Int64, Int64}}:
+ (1, 2)
+ (3, 4)
+ (5, 6)
+ (7, 8)
+```
+"""
 function _reinterpret_unsafe(::Type{T}, x::Vector{R}) where {T,R}
-    # how many T's fit into x?
     @assert isbitstype(T) && isbitstype(R)
+    # how many T's fit into x?
     len = length(x) * sizeof(R)
     p = reinterpret(Ptr{T}, pointer(x))
     return _UnsafeVectorView(0, div(len, sizeof(T)), p)