Add upgrade guide note about potentially breaking change to project r…

…oot `gradle.properties` behavior

See #12021

subprojects/docs/src/docs/userguide/migration/upgrading_version_6.adoc

@@ -63,6 +63,14 @@ If your build fails with a message as above, make sure you configure *both the `
 As a _temporary fix_, you can also disable the new check by setting `-Dorg.gradle.dependency.duplicate.project.detection=false` on the CLI or by adding `systemProp.org.gradle.dependency.duplicate.project.detection=false` to `gradle.properties`.
 This option will be removed in future versions.
 
+==== Properties from project root `gradle.properties` are shared with `buildSrc` and included builds
+
+Before Gradle 6.2, Gradle and project properties set in the `gradle.properties` file in your project root directory were not shared with the `buildSrc` build or included builds.
+
+In Gradle 6.2, Gradle and project properties set in the `gradle.properties` file are shared with the `buildSrc` build and any builds included by the root.
+
+This might cause your build to start failing if the `buildSrc` build or an included build suddenly finds an unexpected or incompatible value for a property coming from the project root `gradle.properties` file. If it does, you can fix it by setting a compatible value to the offending property in a `gradle.properties` file in the failing project directory, as property values set there will take precedence over property values coming from the project root.
+
 === Deprecations
 
 There were no deprecations between Gradle 6.1 and 6.2.