fixup: update the doc for process.exitCode

Signed-off-by: Daeyeon Jeong [email protected]

doc/api/process.md

@@ -1816,9 +1816,15 @@ than the current process.
 
 <!-- YAML
 added: v0.11.8
+changes:
+  - version: REPLACEME
+    pr-url: https://github.com/nodejs/node/pull/43716
+    description: Only accepts a code of type number, or of type string if it
+                 represents an integer.
 -->
 
-* {integer}
+* {integer|string|null|undefined} The exit code. For string type, only
+  integer strings (e.g.,'1') are allowed. **Default:** `undefined`.
 
 A number which will be the process exit code, when the process either
 exits gracefully, or is exited via [`process.exit()`][] without specifying