Bug: notifications.lua 'seen' option is not documented

This fixes #539
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 8248ba5..6412042 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,4 +1,5 @@
 ## Changes in 0.12:
+- Bug: notifications.lua 'seen' option is not documented (#539)
 - Bug: &since option for stats.lua is not documented (#537)
 - Bug: PM setup does not support ES 6 - undocumented (#536)
 - Bug: buffering=1 not allowed with binary files (#534)
diff --git a/docs/API.md b/docs/API.md
index 64f4a5c..7823bec 100644
--- a/docs/API.md
+++ b/docs/API.md
@@ -202,9 +202,10 @@
 
 ### Fetching notifications for a logged in user
 Usage:
-`GET /api/notifications.lua`
+`GET /api/notifications.lua[?seen=$mid]`
 
-Parameters: `None` (cookie required)
+Parameters: (cookie required)
+  - $mid: id of the message to be marked as having been seen
 
 
 Response example:
@@ -213,6 +214,8 @@
 {
     "notifications": {...}
 }
+or
+{"marked": true}
 ~~~
 
 ### Fetching a month's data as an mbox file