Mention validateNoUnrecognizedMessages and remove allWarningsAsErrors

Since aosp/2121872, -Pandroidx.allWarningsAsErrors is no longer used. Let's mention -Pandroidx.validateNoUnrecognizedMessages instead.

PiperOrigin-RevId: 471654068
Change-Id: I79dda9e98de69440ae88373fd6454ac3ab29d3da
diff --git a/docs/onboarding.md b/docs/onboarding.md
index 7fd3c15..2863190 100644
--- a/docs/onboarding.md
+++ b/docs/onboarding.md
@@ -366,7 +366,7 @@
 
 To make warnings fail your build (same as presubmit), use the `--strict` flag,
 which our gradlew expands into a few correctness-related flags including
-`-Pandroidx.allWarningsAsErrors`:
+`-Pandroidx.validateNoUnrecognizedMessages`:
 
 ```shell
 ./gradlew core:core:assemble --strict