Documentation about adding a mutation was hard to follow

I was looking into how to add a "Create User" mutation to my graphql-js server. It took me a while to end up at the right code, so just documenting where I got confused:

1. Searched for "Mutation" on the site. There are two sections ostensibly about mutation:
a. http://graphql.org/learn/queries/#mutations
b. http://graphql.org/learn/schema/#the-query-and-mutation-types
1. Skimming each page, it was hard to figure out how to actually define a mutation in the schema. The code talking about mutations wasn't clear if it was showing how to do them in the query or in the schema.
1. I found the operative sentence is in (b): "Mutations work in a similar way - you define fields on the Mutation type, and those are available as the root mutation fields you can call in your query." and was able to write the correct schema:

```
input CreatePersonInput {
name: String
}
type Mutation {
createPerson(person: CreatePersonInput!): Person
}
```

Suggestions:
- I was thinking it would be better to just put an example of the schema changes necessary to make a mutation right into the schema (b) section to avoid confusion
- Make it clearer when a block of code in the docs is in GraphQL query language or schema language (maybe even show "Query" and "Response" labels next to the left and right sides)

まず graphql.org/learn/queries/#mutations と graphql.org/learn/schema/#the-query-and-mutation-types の mutations セクションを比較します。どの例がクエリ言語で、どの例がスキーマ言語なのかを明確にし、Create User mutation に必要な変更を示すスキーマ例を追加します。読者が両者を区別し、混乱せずにスキーマ定義を見つけられれば完了です。

索引モデルが issue の本文から書いたものです。