Add comments about #strip_markup usage

What does this MR do and why?

Add some code comments to remind developers to use the new #strip_markup helper introduced in https://gitlab.com/gitlab-org/security/gitlab/-/merge_requests/2031.

Part of https://gitlab.com/gitlab-org/gitlab/-/issues/349699

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #349699