Document how to make only some parameters required

[chadfurman]

In response to https://github.com/postgraphql/postgraphql/issues/438

First, test / verify this works. Second, update stored procedure docs to explain how to only make some paraemters required (i.e. not strict) vs all params required (STRICT)

[chadfurman]

@benjie could you assign this to me, also? Helps me keep track of tickets easier -- I can sort all my issues by "recent" and assigned to me :)

[benjie]

👍