Include Trino user header in graceful shutdown docs

[jhcipar]

Description

This PR is a docs-only change intended to address #22593.

I also tried to include a note for additional context based on this comment.

I am trying to learn more about contributing to Trino so apologies in advance if I have missed something glaring from the contributing guidelines.

Additional context and related issues

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:

## Section
* Fix some things. ({issue}`issuenumber`)

[cla-bot]

Thank you for your pull request and welcome to the Trino community. We require contributors to sign our Contributor License Agreement, and we don't seem to have you on file. Continue to work with us on the review and improvements in this PR, and submit the signed CLA to [email protected]. Photos, scans, or digitally-signed PDF files are all suitable. Processing may take a few days. The CLA needs to be on file before we merge your changes. For more information, see https://github.com/trinodb/cla

[jhcipar]

Ah shoot. I sent a CLA but it looks like it hasn't been processed yet. I'll follow up once I get confirmation that it's been processed.

[github-actions]

This pull request has gone a while without any activity. Ask for help on #core-dev on Trino slack.