Add comments to constants.gradle about 'external' version limitations

PiperOrigin-RevId: 383806284

constants.gradle

@@ -21,10 +21,13 @@ project.ext {
     compileSdkVersion = 30
     compileSdkVersion = 30
     dexmakerVersion = '2.28.1'
     dexmakerVersion = '2.28.1'
     junitVersion = '4.13.2'
     junitVersion = '4.13.2'
+    // Use the same Guava version as the Android repo:
+    // https://cs.android.com/android/platform/superproject/+/master:external/guava/METADATA
     guavaVersion = '27.1-android'
     guavaVersion = '27.1-android'
     mockitoVersion = '3.4.0'
     mockitoVersion = '3.4.0'
     mockWebServerVersion = '3.12.0'
     mockWebServerVersion = '3.12.0'
     robolectricVersion = '4.6.1'
     robolectricVersion = '4.6.1'
+    // Keep this in sync with Google's internal Checker Framework version.
     checkerframeworkVersion = '3.5.0'
     checkerframeworkVersion = '3.5.0'
     checkerframeworkCompatVersion = '2.5.0'
     checkerframeworkCompatVersion = '2.5.0'
     jsr305Version = '3.0.2'
     jsr305Version = '3.0.2'