Add missing list of valid backend values.

Author: rlazo

firebase-firestore/firebase-firestore.gradle

@@ -66,6 +66,7 @@ android {
         testInstrumentationRunner "androidx.test.runner.AndroidJUnitRunner"
         consumerProguardFiles 'proguard.txt'
 
+        // Acceptable values are: 'emulator', 'qa', 'nightly', and 'prod'.
         def targetBackend = findProperty("targetBackend") ?: "emulator"
         buildConfigField("String", "TARGET_BACKEND", "\"$targetBackend\"")