Revamp RPC chapter (#35)

* Revamp documentation about transaction RPCs.

* Revamp documentation about Block RPCs.

* Rewrite introduction to RPC chapter. Add new Calling RPCs section.

* Add “RPC requests and responses” section

* Tweak requests and responses section.

* Document request and response types of the state and validator_set RPCs.

* Document the request and response types of the `validator_sets` RPC.

* Document state RPCs.

* Restructure "types featuring in RPC responses" section and add links to it.

* Note that the stake RPC gets from the Next Validator Set network account storage field.

* Add introduction paragraphs for each RPC category.

Author: lyulka

Blockchain.md

@@ -163,7 +163,7 @@ A **transaction** is a digitally signed instruction by an identified party (the
 
 **Receipt** is a type alias of `Vec<CommandReceipt>`, and is a compact summary of what happened during the execution of a transaction.
 
-Each **command receipt** in a receipt describe what happened during each of the transactions' commands:
+Each **command receipt** in a receipt describe what happened during one of the transactions' commands:
 
 |Field|Type|Description|
 |---|---|---|