docs: add missing funcdocs

Author: GNMoseke

Sources/Differentiation/ArrayDifferentiableView+Collection.swift

@@ -10,31 +10,42 @@ extension Array.DifferentiableView:
     @retroactive BidirectionalCollection,
     @retroactive MutableCollection
 where Element: Differentiable {
+    /// The type being stored
     public typealias Element = Array.Element
+
+    /// The type used as an index into the Array
     public typealias Index = Array.Index
+
+    /// A contiguous sequence of values from within the Array
     public typealias SubSequence = Array.SubSequence
 
+    /// Return or set the ``Element`` at the given index
     @inlinable
     public subscript(position: Index) -> Element {
         _read { yield base[position] }
         set(newValue) { base[position] = newValue }
     }
 
+    /// Return or set the ``SubSequence`` at the given range of indices
     @inlinable
     public subscript(bounds: Range<Index>) -> SubSequence {
         get { base[bounds] }
         set(newValue) { base[bounds] = newValue }
     }
 
+    /// The first index of the Array
     @inlinable
     public var startIndex: Index { base.startIndex }
 
+    /// The last index of the Array
     @inlinable
     public var endIndex: Index { base.endIndex }
 
+    /// Create a new ``Array.DifferentiableView``
     @inlinable
     public init() { self.init(Array<Element>()) }
 
+    /// Replace the elements at a range of indices with the values from another ``Collection``
     @inlinable
     public mutating func replaceSubrange<C>(_ subrange: Range<Self.Index>, with newElements: C)
     where C: Collection, Self.Element == C.Element {

Sources/Differentiation/Dictionary+Differentiation.swift

@@ -8,7 +8,11 @@ import _Differentiation
 // https://bugs.swift.org/browse/TF-1193
 
 extension Dictionary: @retroactive Differentiable where Value: Differentiable {
+    /// Tangent Vectors for each key-value pair
     public typealias TangentVector = [Key: Value.TangentVector]
+
+    /// Shift the dictionary by the given ``TangentVector``
+    /// All keys in the given shift vectar must exist in the dictionary
     public mutating func move(by direction: TangentVector) {
         for (componentKey, componentDirection) in direction {
             func fatalMissingComponent() -> Value {
@@ -18,6 +22,7 @@ extension Dictionary: @retroactive Differentiable where Value: Differentiable {
         }
     }
 
+    /// Create a zero tangent vector for every key
     public var zeroTangentVectorInitializer: () -> TangentVector {
         let listOfKeys = keys  // capturing only what's needed, not the entire self, in order to not waste memory
         func initializer() -> Self.TangentVector {
@@ -29,14 +34,17 @@ extension Dictionary: @retroactive Differentiable where Value: Differentiable {
 
 /// Implements the `AdditiveArithmetic` requirements.
 extension Dictionary: @retroactive AdditiveArithmetic where Value: AdditiveArithmetic {
+    /// Combine two dictionaries by adding their keys
     public static func + (_ lhs: Self, _ rhs: Self) -> Self {
         lhs.merging(rhs, uniquingKeysWith: +)
     }
 
+    /// Combine two dictionaries by subtracting their keys
     public static func - (_ lhs: Self, _ rhs: Self) -> Self {
         lhs.merging(rhs.mapValues { .zero - $0 }, uniquingKeysWith: +)
     }
 
+    /// Return an empty dictionary
     public static var zero: Self { [:] }
 }
 

Sources/Differentiation/Optional+DifferentiableMap.swift

@@ -3,6 +3,7 @@
 import _Differentiation
 
 extension Optional where Wrapped: Differentiable {
+    /// Apply a differentiable closure to the wrapped value
     @inlinable
     @differentiable(reverse,wrt: self)
     public func differentiableMap<Result: Differentiable>(