docs: update the comment regarding eligible SQL shapes for PartitionQ…

…uery

PiperOrigin-RevId: 602806739

google/spanner/v1/spanner.proto

@@ -928,9 +928,10 @@ message PartitionQueryRequest {
 
   // Required. The query request to generate partitions for. The request will
   // fail if the query is not root partitionable. For a query to be root
-  // partitionable, it needs to satisfy a few conditions. For example, the first
-  // operator in the query execution plan must be a distributed union operator.
-  // For more information about other conditions, see [Read data in
+  // partitionable, it needs to satisfy a few conditions. For example, if the
+  // query execution plan contains a distributed union operator, then it must be
+  // the first operator in the plan. For more information about other
+  // conditions, see [Read data in
   // parallel](https://cloud.google.com/spanner/docs/reads#read_data_in_parallel).
   //
   // The query request must not contain DML commands, such as INSERT, UPDATE, or