doc: edit stability text for clarity and style

PR-URL: https://github.com/nodejs/node/pull/11112
Reviewed-By: Timothy Gu <timothygu99@gmail.com>
Reviewed-By: Italo A. Casas <me@italoacasas.com>
Reviewed-By: Claudio Rodriguez <cjrodr@yahoo.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>

doc/api/documentation.md

@@ -58,7 +58,7 @@ is a high priority, and will not be broken unless absolutely necessary.
 
 ```txt
 Stability: 3 - Locked
-Only fixes related to security, performance, or bug fixes will be accepted.
+Only bug fixes, security fixes, and performance improvements will be accepted.
 Please do not suggest API changes in this area; they will be refused.
 ```