Update mdbook to reflect "call" method changes

[maackle]

Partially complete update to mdbook to reflect changes to call RPC method. Still needs to be updated in two subsections.

I also propose:

• we rename "params" to "args" or "arguments", to avoid nested the nested "params".
• we allow both "params" and "args" in the "call" method handler, deprecating "params".
• then we update the documentation to use "args".

Also, params is a misnomer here anyway. According to generally accepted notions, "parameters" are the placeholders you put in a function definition, and "arguments" are the actual values you pass when invoking a function, and the latter is what we're doing here.

This is what I'm talking about

{
    "jsonrpc": "2.0",
    "id": "0",
    "method": "call",
    "params": {  // <--------- from JSON-RPC spec
        "instance_id": "test-instance",
        "zome": "blog",
        "function": "create_blog",
        "params": {  // <---------- proposing we change to "args"
            "blog": {
                "content": "sample content"
            } 
        }
    }
}

• my changes to the code affect some exposed aspect of the developer experience, and I have added an item to relevant 'Added, Fixed, Changed, Removed, Deprecated, or Security' heading under the 'Unreleased' heading of the CHANGELOG, with the format - summary of change [PR#1234](https://github.com/holochain/holochain-rust/pull/1234)

[Connoropolous]

@maackle I also really disliked the params -> params
so I am very in favor of updating to using a key called args 👍

[zippy]

I'm ok with doing this update, but if we are going to do it, lets convert this PR/branch into not just a documentation change, but do the work here too. Also we should do it soon. Someone wanna take this on and add into the tree?

[maackle]

I can definitely take it on next week, and maybe this week.

We can bundle up #1191 with this work as well.

[maackle]

But, @zippy if by "soon" you mean this week, maybe someone else can take it on

[zippy]

Hey @dymayday how about you take these on!?

[dymayday]

Sure! I will be more than happy to do it as soon as I'm done with the guidebook, I don't want to mess with my first PR...