This clarifies the usage of req.params, including an anti-pattern example that will cause unexpected behavior.
Clarify modification of req.params
This applies more to req.params than app.param(), not sure this belongs in this section.
IMO it is sufficient to mention in the req.params section that the object is read only.
Does not belong to app.param. Also, there are many other objects, a developer might try to set properties on. Cannot add them all, unless a decision is made about such possibilities.