Clarify that route defaults don't need a placeholder
feature #4017 Clarify that route defaults don't need a placeholder (iamdto)

This PR was merged into the 2.3 branch.

Discussion
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Clarify that route defaults don't need a placeholder

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | 2.3+
| Fixed tickets | #3960

Commits
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a18e5cc Clarify that route defaults don't need a placeholder