Document `/help` tests

What does this MR do?

Documents how /help pages tests work, and gives a caution.

Related issue?

https://gitlab.com/gitlab-org/gitlab-ce/issues/54211

Author's checklist

• Apply the correct labels and milestone
• Port the MR to EE (or backport from CE): always recommended, required when the ee-compat-check job fails

Review checklist

• Your team's review (required)
• PM's review (recommended, but not a blocker)
• Technical writer's review (required)
• Merge the EE-MR first, CE-MR afterwards

/label ~Documentation