Make supported bundler version clearer and polish topic

What does this MR do?

Primarily to highlight we want to install a version of bundler lower than v2.

While here, implemented some newer styles and conventions, and gave some sentences an edit.

Related issues

Relates to: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/24502.

Author's checklist

• Apply the correct labels and milestone

Review checklist

• Your team's review (required)
• Technical writer's review (required)