Update KSBackingFieldJavaImpl.docString to refer to parent property's doc string

kotlin-analysis-api/src/main/kotlin/com/google/devtools/ksp/impl/symbol/kotlin/KSBackingFieldJavaImpl.kt

diff --git a/kotlin-analysis-api/src/main/kotlin/com/google/devtools/ksp/impl/symbol/kotlin/KSBackingFieldJavaImpl.kt b/kotlin-analysis-api/src/main/kotlin/com/google/devtools/ksp/impl/symbol/kotlin/KSBackingFieldJavaImpl.kt
index ddb9cca..b10e3a4 100644
--- a/kotlin-analysis-api/src/main/kotlin/com/google/devtools/ksp/impl/symbol/kotlin/KSBackingFieldJavaImpl.kt
+++ b/kotlin-analysis-api/src/main/kotlin/com/google/devtools/ksp/impl/symbol/kotlin/KSBackingFieldJavaImpl.kt
@@ -70,6 +70,10 @@
         }
     }
 
+    override val docString: String? by lazy {
+        property.docString
+    }
+
     // Manual delegation for KSExpectActual to avoid eager evaluation in the class header
     private val expectActualImpl by lazy { KSExpectActualImpl(ktJavaFieldSymbol) }
     override val isActual: Boolean get() = expectActualImpl.isActual