Clarify generatedStubClasses meaning

[agluszak]

generatedStubClasses does not appear to drive behavior

• The field still exists in the protobuf schema at src/main/protobuf/kotlin_model.proto:107 and KotlinBuilder still populates it at src/main/kotlin/io/bazel/kotlin/builder/tasks/KotlinBuilder.kt:276.
• The actual stub path used by plugin expansion comes from temp/stubs via src/main/kotlin/io/bazel/kotlin/builder/tasks/jvm/CompilationTask.kt:436, not from generatedStubClasses.
• The KAPT stub jar is built from incrementalData, and the generated class jar is built from generatedClasses, at src/main/kotlin/io/bazel/kotlin/builder/tasks/jvm/CompilationTask.kt:309 and src/main/kotlin/io/bazel/kotlin/builder/tasks/jvm/CompilationTask.kt:324.