[API] add sub/unsub docs

mkdocs/docs/api/channel.md

@@ -7,7 +7,7 @@ Parameter:
 
 - filter: subscribed
 
-Subscribe to a list of channels:  
+Subscribe/Unsubscribe to a list of channels:  
 **POST** `/api/channel/`
 ```json
 {

mkdocs/docs/api/playlist.md

@@ -3,6 +3,16 @@
 ## Playlist List
 **GET** `/api/playlist/`
 
+Subscribe/Unsubscribe to a list of playlists:  
+**POST** `/api/playlist/`
+```json
+{
+    "data": [
+        {"playlist_id": "PL96C35uN7xGLLeET0dOWaKHkAlPsrkcha", "playlist_subscribed": true}
+    ]
+}
+```
+
 ## Playlist Item
 **GET** `/api/playlist/<playlist_id>/`