docs(unit-testing): explain `overrideMiddleware`

[schiemon]

PR Checklist

Please check if your PR fulfills the following requirements:

• [x] The commit message follows our guidelines: https://github.com/nestjs/docs.nestjs.com/blob/master/CONTRIBUTING.md

PR Type

What kind of change does this PR introduce?

• [ ] Bugfix
• [ ] Feature
• [ ] Code style update (formatting, local variables)
• [ ] Refactoring (no functional changes, no api changes)
• [ ] Build related changes
• [x] Docs
• [ ] Other... Please describe:

This PR extends the documentation for the overrideMiddleware functionality described in this PR.

Does this PR introduce a breaking change?

• [ ] Yes
• [x] No