docgen: update BEGIN and SET TRANSACTION diagrams

[taroface]

Updated the BEGIN diagram, which had been empty since this commit. It now also shows the ISOLATION LEVEL syntax:

Also updated SET TRANSACTION with the ISOLATION LEVEL syntax. The diagram had also not been displaying with 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 

[yuzefovich]

Thanks!

Reviewed 10 of 10 files at r1, all commit messages.
Reviewable status: complete! 1 of 0 LGTMs obtained (waiting on @michae2)

[taroface]

bors r=yuzefovich

[craig]

Build succeeded:

• unit_tests
• acceptance
• linux_arm64_build
• local_roachtest
• local_roachtest_fips
• macos_arm64_build
• check_generated_code
• linux_amd64_fips_build
• lint
• linux_amd64_build
• examples_orms
• windows_build
• docker_image_amd64
• macos_amd64_build