Fix docs

Author: Zhen Li

driver/src/main/java/org/neo4j/driver/v1/Value.java

@@ -240,6 +240,7 @@ public interface Value extends MapAccessor, MapAccessorWithDefaultValue
     /**
      * @param defaultValue return this value if the value is null.
      * @return the value as a Java String, if possible
+     * @throws Uncoercible if value types are incompatible.
      */
     String asString( String defaultValue );