Update docs and example

docs/en/joinus/README.md

@@ -270,11 +270,17 @@ By default there is a global caching period set in `lib/config.js`, some sources
 -   Save to cache:
 
 ```js
-ctx.cache.set((key: string), (value: string), (time: number)); // time is the caching period in seconds
+ctx.cache.set((key: string), (value: string), (time: number), (persistent: boolean)); // time is the caching period in seconds. When persistent is set to true, the cache will refresh its expiry time upon every re-request (Under Redis this means the cache will never expire).
 ```
 
 -   Access the cache:
 
+::: tip tips
+
+Since Redis caches with `persistent = true` will never expire, parameter `persistent` for `Get()` only has effect on LRU memory cache, it serves no purpose for Redis.
+
+:::
+
 ```js
 const value = await ctx.cache.get((key: string));
 ```
@@ -285,19 +291,19 @@ Given the update frequency is known, set the appropriate caching period to reuse
 
 ```js
 const key = 'daily' + story.id; // story.id is the unique identifier of each article
-ctx.cache.set(key, item.description, 24 * 60 * 60); // set the caching period to 24 hours * 60 minutes * 60 seconds = 86,400 seconds = 1 day
+ctx.cache.set(key, item.description, 24 * 60 * 60, true); // set the LRU memeroy caching period to 24 hours * 60 minutes * 60 seconds = 86,400 seconds = 1 day
 ```
 
 When the identical requests come in, reuse the cache：
 
 ```js
 const key = 'daily' + story.id;
-const value = await ctx.cache.get(key); // query the cache to find the unique identifier
+const value = await ctx.cache.get(key, true); // query the LRU memory cache to find the unique identifier
 if (value) {
     // return the cached data
     item.description = value; // assign the cached data
 } else {
-    // no cached found
+    // no cache found
     // initiate request to the data source
 }
 ```

docs/joinus/README.md

@@ -267,29 +267,35 @@ sidebar: auto
 -   添加缓存:
 
 ```js
-ctx.cache.set((key: string), (value: string), (time: number)); // time 为缓存时间, 单位为秒
+ctx.cache.set((key: string), (value: string), (time: number), (persistent: boolean)); // time 为缓存时间。单位为秒。persistent 为 true 则为缓存自动延长过期时间 (Redis 下则为永不过期)。
 ```
 
 -   获取缓存:
 
+::: tip 提示
+
+ 由于 Redis 下 `persistent = true` 的缓存不会过期，`Get()` 下 `persistent` 参数仅适用于 LRU memory 缓存，对于 Redis 不会有任何效果。
+
+:::
+
 ```js
 const value = await ctx.cache.get((key: string));
 ```
 
 例如知乎日报需要获取文章全文：[/lib/routes/zhihu/daily.js](https://github.com/DIYgod/RSSHub/blob/master/lib/routes/zhihu/daily.js), 每篇文章都需要单独请求一次。
 
-由于已知文章更新频率为一天，把结果缓存一天，可以让后续的请求直接使用已缓存的数据，从而提升性能并节省资源。
+由于已知文章更新频率为一天，把结果缓存一天，可以让后续的请求直接使用已缓存的数据，并且该缓存被再次访问后会延长过期时间，从而提升性能并节省资源。
 
 ```js
 const key = 'daily' + story.id; // story.id 为知乎日报返回的文章唯一识别符
-ctx.cache.set(key, item.description, 24 * 60 * 60); // 设置缓存时间为 24小时 * 60分钟 * 60秒 = 86400秒 = 1天
+ctx.cache.set(key, item.description, 24 * 60 * 60, true); // 设置 LRU memory 缓存时间为 24小时 * 60分钟 * 60秒 = 86400秒 = 1天
 ```
 
 当同样的请求被发起时，优先使用未过期的缓存：
 
 ```js
 const key = 'daily' + story.id;
-const value = await ctx.cache.get(key); // 从缓存中查询是否已存在该文章唯一识别符
+const value = await ctx.cache.get(key, true); // 从 LRU memory 缓存中查询是否已存在该文章唯一识别符
 if (value) {
     // 查询返回未过期缓存
     item.description = value; // 直接赋值

lib/routes/zhihu/daily.js

@@ -24,7 +24,7 @@ module.exports = async (ctx) => {
                 link: 'https://news-at.zhihu.com/story/' + story.id,
             };
             const key = 'daily' + story.id;
-            const value = await ctx.cache.get(key);
+            const value = await ctx.cache.get(key, true);
 
             if (value) {
                 item.description = value;
@@ -37,7 +37,7 @@ module.exports = async (ctx) => {
                     },
                 });
                 item.description = utils.ProcessImage(storyDetail.data.body.replace(/<div class="meta">([\s\S]*?)<\/div>/g, '<strong>$1</strong>').replace(/<h2.*?>/g, ''));
-                ctx.cache.set(key, item.description, 24 * 60 * 60);
+                ctx.cache.set(key, item.description, 24 * 60 * 60, true);
             }
 
             return Promise.resolve(item);