More specific guidance around BuildCompat checks

PiperOrigin-RevId: 529735932
Change-Id: I050ff834fc5ed4facf38be11fd8eb295061c0dec

Author: AndroidX Core Team

docs/api_guidelines/compat.md

@@ -56,7 +56,13 @@ private object Api29Impl {
 ```
 
 When developing against pre-release SDKs where the `SDK_INT` has not been
-finalized, SDK checks **must** use `BuildCompat.isAtLeastX()` methods.
+finalized, SDK checks **must** use `BuildCompat.isAtLeastX()` methods and
+**must** use a tip-of-tree `project` dependency to ensure that the
+implementation of `BuildCompat` stays up-to-date when the SDK is finalized.
+
+**Do not** assume that the next SDK release's `SDK_INT` will be N+1. The value
+is not finalized until SDK finalization happens, at which point the `isAtLeast`
+check will be updated. **Never** write your own check for a pre-release SDK.
 
 ```java {.good}
 @NonNull
@@ -68,6 +74,12 @@ public static List<Window> getAllWindows() {
 }
 ```
 
+```kotlin {.good}
+dependencies {
+  api(project(":core:core"))
+}
+```
+
 ##### Preventing invalid casting {#compat-casting}
 
 Even when a call to a new API is moved to a version-specific class, a class