Add scaladoc

cache/src/main/scala/coursier/CachePolicy.scala

@@ -3,11 +3,56 @@ package coursier
 sealed abstract class CachePolicy extends Product with Serializable
 
 object CachePolicy {
+
+  /** Only pick local files, possibly from the cache. Don't try to download anything. */
   case object LocalOnly extends CachePolicy
+
+  /**
+    * Only pick local files. If one of these local files corresponds to a changing artifact, check
+    * for updates, and download these if needed.
+    *
+    * If no local file is found, *don't* try download it. Updates are only checked for files already
+    * in cache.
+    */
   case object LocalUpdateChanging extends CachePolicy
+
+  /**
+    * Only pick local files, check if any update is available for them, and download these if needed.
+    *
+    * If no local file is found, *don't* try download it. Updates are only checked for files already
+    * in cache.
+    *
+    * Unlike `LocalUpdateChanging`, all found local files are checked for updates, not just the
+    * changing ones.
+    */
   case object LocalUpdate extends CachePolicy
+
+  /**
+    * Pick local files, and download the missing ones.
+    *
+    * For changing ones, check for updates, and download those if any.
+    */
   case object UpdateChanging extends CachePolicy
+
+  /**
+    * Pick local files, download the missing ones, check for updates and download those if any.
+    *
+    * Unlike `UpdateChanging`, all found local files are checked for updates, not just the changing
+    * ones.
+    */
   case object Update extends CachePolicy
+
+  /**
+    * Pick local files, download the missing ones.
+    *
+    * No updates are checked for files already downloaded.
+    */
   case object FetchMissing extends CachePolicy
+
+  /**
+    * (Re-)download all files.
+    *
+    * Erases files already in cache.
+    */
   case object ForceDownload extends CachePolicy
 }