Add information about no primary key spanner table

Author: itsankit-google

docs/Spanner-batchsink.md

@@ -42,6 +42,10 @@ Each record is composed of columns (also called fields).
 Every table is defined by a schema that describes the column names, data types, and other information.
 If the table does not exist, it will get created.
 
+**Note**: A table with no primary key columns can have only one row.
+Only GoogleSQL-dialect databases can have tables without a primary key.
+Please click [here](https://cloud.google.com/spanner/docs/schema-and-data-model) for more details.
+
 **Primary Key**: If the table does not exist, a primary key must be provided in order to auto-create the table.
 The key can be a composite key of multiple fields in the schema. This is not required if the table already exists.
 

src/main/java/io/cdap/plugin/gcp/spanner/sink/SpannerSinkConfig.java

@@ -55,7 +55,10 @@ public class SpannerSinkConfig extends PluginConfig {
   private static final String NAME_CMEK_KEY = "cmekKey";
 
   @Name(NAME_TABLE)
-  @Description("Cloud Spanner table id. Uniquely identifies your table within the Cloud Spanner database")
+  @Description("Cloud Spanner table id. Uniquely identifies your table within the Cloud Spanner database."
+      + " Note: A table with no primary key columns can have only one row."
+      + " Only GoogleSQL-dialect databases can have tables without a primary key."
+      + " Please refer to https://cloud.google.com/spanner/docs/schema-and-data-model for more details.")
   @Macro
   private String table;