doc: make the NODE_VERSION_IS_RELEASE revert clear

PR-URL: nodejs/node#49114
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Yagiz Nizipli <yagiz@nizipli.com>
Reviewed-By: Chemi Atlow <chemi@atlow.co.il>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>

graal-nodejs/doc/contributing/releases.md

@@ -813,7 +813,9 @@ $ git commit --amend
 </details>
 
 Even if there are no conflicts, ensure that you revert all the changes that were
-made to `src/node_version.h`.
+made to `src/node_version.h`. `NODE_VERSION_IS_RELEASE` must be `0`.
+
+<sup>Edit `src/node_version.h`, revert `NODE_VERSION_IS_RELEASE` back to `0`, and `git commit --amend`</sup>
 
 If there are conflicts in `doc` due to updated `REPLACEME`
 placeholders (that happens when a change previously landed on another release