chore: make Rehydratable use Awaitable; add docblock

src/util/support/Rehydratable.ts

@@ -2,6 +2,7 @@
  * Type representing a JSON serializable object.
  */
 import {ErrorWithContext} from '../error/ErrorWithContext'
+import {Awaitable} from './types'
 
 export type JSONState = { [key: string]: string | boolean | number | undefined | JSONState | Array<string | boolean | number | undefined | JSONState> }
 
@@ -30,14 +31,14 @@ export function isJSONState(what: unknown): what is JSONState {
 export interface Rehydratable {
     /**
      * Dehydrate this class' state and get it.
-     * @return Promise<JSONState>
+     * @return JSONState|Promise<JSONState>
      */
-    dehydrate(): Promise<JSONState>
+    dehydrate(): Awaitable<JSONState>
 
     /**
      * Rehydrate a state into this class.
      * @param {JSONState} state
      * @return void|Promise<void>
      */
-    rehydrate(state: JSONState): void | Promise<void>
+    rehydrate(state: JSONState): Awaitable<void>
 }