docgen: update BEGIN and SET TRANSACTION diagrams without iso level

[taroface]

Updated the BEGIN diagram for v23.1, which had been empty since this commit. (#137058 covers v23.2 and later.)

Also updated the SET TRANSACTION diagram, which had not been displaying most of its syntax.

Epic: none
Release note: none
Release justification: non-production code change

[blathers-crl]

It looks like your PR touches production code but doesn't add or edit any test code. Did you consider adding tests to your PR?

It looks like your PR touches SQL parser code but doesn't add or edit parser tests. Please make sure you add or edit parser tests if you edit the parser.

_{🦉 Hoot! I am a Blathers, a bot for CockroachDB. My owner is dev-inf.}

[cockroach-teamcity]

This change is 

[taroface]

blathers backport 23.1

[blathers-crl]

Encountered an error creating backports. Some common things that can go wrong:

1. The backport branch might have already existed.
2. There was a merge conflict.
3. The backport branch contained merge commits.

You might need to create your backport manually using the backport tool.

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error creating merge commit from 4ed4c5c to blathers/backport-release-23.1-137368: POST https://api.github.com/repos/cockroachdb/cockroach/merges: 409 Merge conflict []

you may need to manually resolve merge conflicts with the backport tool.

Backport to branch 23.1 failed. See errors above.

---

_{🦉 Hoot! I am a Blathers, a bot for CockroachDB. My owner is dev-inf.}

[taroface]

Closing as this was opened to create #137371