Mention validateNoUnrecognizedMessages and remove allWarningsAsErrors
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

AndroidX/androidx,androidx/androidx,androidx/androidx,androidx/androidx,AndroidX/androidx,AndroidX/androidx,androidx/androidx,AndroidX/androidx,androidx/androidx,androidx/androidx,androidx/androidx,androidx/androidx,AndroidX/androidx,AndroidX/androidx,AndroidX/androidx,androidx/androidx,AndroidX/androidx,AndroidX/androidx,androidx/androidx,AndroidX/androidx