Update release notes to better describe Java 8 change.

Apps need to set the target compatibility to VERSION_1_8 to enable the
automatic desugaring if they haven't done so already.

Issue:#4907

-------------
Created by MOE: https:/google/moe
MOE_MIGRATED_REVID=215733070

RELEASENOTES.md

@@ -25,9 +25,8 @@
 
 ### 2.9.0 ###
 
-* Turn on Java 8 compiler support for the ExoPlayer library. Apps that depend
- on ExoPlayer via its source code rather than an AAR may need to add
- `compileOptions { targetCompatibility JavaVersion.VERSION_1_8 }` to their
+* Turn on Java 8 compiler support for the ExoPlayer library. Apps may need to
+ add `compileOptions { targetCompatibility JavaVersion.VERSION_1_8 }` to their
  gradle settings to ensure bytecode compatibility.
 * Set `compileSdkVersion` and `targetSdkVersion` to 28.
 * Support for automatic audio focus handling via