docs: update tag descriptions

[MikeMcC399]

• Second stage in implementation of enhancement from issue Support Renovate updating of Cypress Docker images #1255
• Follows on from PR feat: add BROWSERS_IMAGE_SHORT_TAG #1256

Issue

• PR feat: add BROWSERS_IMAGE_SHORT_TAG #1256 adds the BROWSERS_IMAGE_TAG used for the cypress/browsers Docker image

Change

• Add BROWSERS_IMAGE_TAG and rework tag documentation.

[cypress-app-bot]

• Create a Draft Pull Request if your PR is not ready for review. Mark the PR as Ready for Review when you're ready for a Cypress team member to review the PR.

[MikeMcC399]

• cypress/browsers:22.11.0 successfully tested in PR test: use short-form cypress/browsers tag github-action#1318