docs(samples): add tag to statement timeout sample (#2931)

* docs(samples): add tag to statement timeout sample

Add a tag to the StatementTimeoutExample, so we can include this sample
in the documentation.

* docs: add clarifying comment regarding setting longer timeout

* chore: reduce line lengths

samples/snippets/src/main/java/com/example/spanner/StatementTimeoutExample.java

@@ -32,6 +32,8 @@
 
 class StatementTimeoutExample {
 
+  // [START spanner_set_statement_timeout]
+
   static void executeSqlWithTimeout() {
     // TODO(developer): Replace these variables before running the sample.
     String projectId = "my-project";
@@ -52,6 +54,10 @@ public <ReqT, RespT> ApiCallContext configure(ApiCallContext context, ReqT reque
           MethodDescriptor<ReqT, RespT> method) {
         // DML uses the ExecuteSql RPC.
         if (method == SpannerGrpc.getExecuteSqlMethod()) {
+          // NOTE: You can use a GrpcCallContext to set a custom timeout for a single RPC
+          // invocation. This timeout can however ONLY BE SHORTER than the default timeout
+          // for the RPC. If you set a timeout that is longer than the default timeout, then
+          // the default timeout will be used.
           return GrpcCallContext.createDefault()
               .withCallOptions(CallOptions.DEFAULT.withDeadlineAfter(60L, TimeUnit.SECONDS));
         }
@@ -73,4 +79,5 @@ public <ReqT, RespT> ApiCallContext configure(ApiCallContext context, ReqT reque
         })
     );
   }
+  // [END spanner_set_statement_timeout]
 }