Add missing API docs

Author: ra1028

Sources/Atoms/Atom/Atom.swift

@@ -12,6 +12,7 @@ public protocol Atom {
     /// A loader type that represents an actual implementation of the corresponding atom.
     associatedtype Loader: AtomLoader where Loader.Coordinator == Coordinator
 
+    /// A type to coordinate with the atom.
     associatedtype Coordinator = Void
 
     /// A type of the context structure that to read, watch, and otherwise interacting
@@ -33,6 +34,13 @@ public protocol Atom {
     /// If this atom conforms to `Hashable`, it will adopt itself as the `key` by default.
     var key: Key { get }
 
+    /// Creates the custom coordinator instance that you use to preserve arbitrary state of
+    /// the atom.
+    ///
+    /// It's called when the atom is initialized, and the same instance is preserved until
+    /// the atom is no longer used and is deinitialized.
+    ///
+    /// - Returns: The atom's associated coordinator instance.
     func makeCoordinator() -> Coordinator
 
     // --- Internal ---
@@ -47,9 +55,13 @@ public extension Atom {
         false
     }
 
-    func makeCoordinator() -> Coordinator where Coordinator == Void { () }
+    func makeCoordinator() -> Coordinator where Coordinator == Void {
+        ()
+    }
 }
 
 public extension Atom where Self == Key {
-    var key: Self { self }
+    var key: Self {
+        self
+    }
 }

Sources/Atoms/Core/Loader/AtomLoader.swift

@@ -4,6 +4,7 @@ public protocol AtomLoader {
     /// A type of value to provide.
     associatedtype Value
 
+    /// A type to coordinate with the atom.
     associatedtype Coordinator
 
     /// The context structure that to interact with an atom store.