docs: add "incompatible immutable parameters" note to parameters doc

relates to https://github.com/coder/coder/pull/18365

when 1. a required immutable parameter is empty, or 2. an immutable parameter, required or not, has a validation error or warning the Parameters page shows the “Some parameters are incompatible” error and blocks out the **Save** button until issues are resolved

https://coder.com/docs/admin/templates/extending-templates/parameters