Document sort direction and null order in Iceberg

[okumin]

Description

A trivial doc update to describe how users can explicitly specify sort directions or null ordering when we create a sorted table of Apache Iceberg.

Additional context and related issues

We found no Trino document mentioning the advanced parameters of sorted_by while developing Apache Hive. Simply, we misunderstood that Trino would not have the capability at first glance. I believe some people would love the information.

Release notes

(x) This is not user-visible or is docs only, and no release notes are required.
( ) Release notes are required. Please propose a release note for me.
( ) Release notes are required, with the following suggested text:

[okumin]

I tested this SQL in my local machine.

trino> CREATE TABLE iceberg.default.orders (
    ->     order_id BIGINT,
    ->     order_date DATE,
    ->     account_number BIGINT,
    ->     customer VARCHAR,
    ->     country VARCHAR)
    -> WITH (sorted_by = ARRAY['order_date DESC NULLS FIRST', 'order_id ASC NULLS LAST'])
    -> ;
CREATE TABLE
trino> 

$ hdfs dfs -cat /user/hive/warehouse/orders-5fae815b97d54add91c0100370e56f0c/metadata/00000-cf46fb4f-226b-42dc-b640-c14d9e000c09.metadata.json
...
  "schemas" : [ {
    "type" : "struct",
    "schema-id" : 0,
    "fields" : [ {
      "id" : 1,
      "name" : "order_id",
      "required" : false,
      "type" : "long"
    }, {
      "id" : 2,
      "name" : "order_date",
      "required" : false,
      "type" : "date"
    }, {
      "id" : 3,
      "name" : "account_number",
      "required" : false,
      "type" : "long"
    }, {
      "id" : 4,
      "name" : "customer",
      "required" : false,
      "type" : "string"
    }, {
      "id" : 5,
      "name" : "country",
      "required" : false,
      "type" : "string"
    } ]
  } ],
...
  "default-sort-order-id" : 1,
  "sort-orders" : [ {
    "order-id" : 1,
    "fields" : [ {
      "transform" : "identity",
      "source-id" : 2,
      "direction" : "desc",
      "null-order" : "nulls-first"
    }, {
      "transform" : "identity",
      "source-id" : 1,
      "direction" : "asc",
      "null-order" : "nulls-last"
    } ]
  } ],

[okumin]

This is the part I updated

[okumin]

Thanks. I made two changes.

1. Accepted your suggestion
2. Rebase the two commits into a single commit, rephrasing the commit message briefly

[okumin]

Thanks for reviewing and merging my PR. It will hopefully help future users 🙏