Add "x" event to the asciicast v3 spec

Author: ku1ik

docs/manual/asciicast/v3.md

@@ -31,6 +31,7 @@ asciicast v3 file looks like this:
 # terminal window resized to 90 cols and 30 rows
 [2.050000, "r", "90x30"]
 [1.541828, "o", "Bye!"]
+[0.8870, "x", "0"]
 ```
 
 ## Header
@@ -204,8 +205,8 @@ Each element of the event stream is a 3-tuple encoded as JSON array:
 Where:
 
 * `interval` (float) - time interval from the previous event (in seconds),
-* `code` (string) - event type code, one of: `"o"`, `"i"`, `"m"`, `"r"`
-* `data` (any) - event specific data, described separately for each event
+* `code` (string) - event type code, one of: `"o"`, `"i"`, `"m"`, `"r"`, `"x"`
+* `data` (string) - event specific data, described separately for each event
   code.
 
 For example, let's look at the following line:
@@ -227,7 +228,7 @@ current and future versions of the format. For example, we may add new event
 code for text overlay (subtitles display).
 
 A tool which interprets the event stream (a player or a post-processor) should
-ignore (or pass through) event codes it doesn't understand or doesn't care
+ignore (or pass through) event with codes it doesn't understand or doesn't care
 about.
 
 #### o - output, data written to a terminal
@@ -294,10 +295,28 @@ Those are captured in response to `SIGWINCH` signal.
 `data` contains new terminal size (columns + rows) formatted as
 `"{COLS}x{ROWS}"`.
 
+Example:
+
 ```json
 [5.0, "r", "100x50"]
 ```
 
+### x - exit
+
+Event with code `"x"` represents session exit status.
+
+`data` is a numerical exit status of session's main child process (e.g. a
+shell).
+
+This event is typically included only once, as the very last event of the
+recording.
+
+Example:
+
+```json
+[5.0, "x", "0"]
+```
+
 ## File extension
 
 Suggested file extension is `.cast`.