tweak docs

docs/realm.js

@@ -107,7 +107,8 @@ class Realm {
  * Get the current schema version of the Realm at the given path.
  * @param {string} path - The path to the file where the
  *   Realm database is stored.
- * @param {string} encryptionKey - Optional encryption key.
+ * @param {ArrayBuffer|ArrayBufferView} [encryptionKey] - Required only when
+ *   accessing encrypted Realms.
  * @throws {Error} When passing an invalid or non-matching encryption key.
  * @returns {number} version of the schema, or `-1` if no Realm exists at `path`.
  */