doc: update umask for clarity

PR-URL: #14170
Fixes: #14169
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

Author: jsumners

doc/api/process.md

@@ -1654,9 +1654,9 @@ added: v0.1.19
 * `mask` {number}
 
 The `process.umask()` method sets or returns the Node.js process's file mode
-creation mask. Child processes inherit the mask from the parent process. The old
-mask is return if the `mask` argument is given, otherwise returns the current
-mask.
+creation mask. Child processes inherit the mask from the parent process. Invoked
+without an argument, the current mask is returned, otherwise the umask is set to
+the argument value and the previous mask is returned.
 
 ```js
 const newmask = 0o022;