Document parameter defaults

[ddeaguiar]

Description

Let's make this more visible in the docs.

[ddeaguiar]

A recent commit adds a note regarding default parameters in the Handling Parameters section of the Your First API guide but I still think we should include this documentation elsewhere as well. Perhaps the Action Literals document?

[ohpauleez]

Default parameters were added in the lastest evolution of Vase, which is why the documentation is light. I agree, we should mention it in the docs (the samples and tests illustrate it)

[mtnygard]

I think we should add a new doc that is all about parameters. There is more subtlety that we should cover:

• Dealing with namespaced names from JSON
• The order of merging params. Who wins if there is a query-param, a path-param, and a body-param with the same key?
• The role of edn-coerce.