Better naming for default value of parameters and relevant doc refine

[whynowy]

Is your feature request related to a problem? Please describe.

1. Default value of a parameter can be specified with a key value, it's better to use a key like default;
2. Dedicated docs for parameters feature instead of only can be found in the tutorials.

[jeremyj563]

I'm also confused about how to use a default value that is a type other than string. Is it even possible currently?