doc: make os api doc more consistent

This adds a missing Returns to os.arch() as well as a missing added in
version to os.constants.

PR-URL: https://github.com/nodejs/node/pull/10994
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

doc/api/os.md

@@ -26,6 +26,8 @@ A string constant defining the operating system-specific end-of-line marker:
 added: v0.5.0
 -->
 
+* Returns: {String}
+
 The `os.arch()` method returns a string identifying the operating system CPU
 architecture *for which the Node.js binary was compiled*.
 
@@ -36,6 +38,9 @@ The current possible values are: `'arm'`, `'arm64'`, `'ia32'`, `'mips'`,
 Equivalent to [`process.arch`][].
 
 ## os.constants
+<!-- YAML
+added: v6.3.0
+-->
 
 * {Object}