Improve docs: add more details for JSON schema validation

[radomird]

Prerequisites

• I have written a descriptive issue title
• I have searched existing issues to ensure the issue has not already been raised

Issue

When validating non-file fields in the body and NOT using the shared schema, according to the plugin documentation, we can validate the fields by using the following syntax:

body: {
  type: 'object',
  properties: {
    hello: {
      properties: {
        value: { 
          type: 'string',
        }
      }
    }
 }

However one would expect that for the non-file fields (like this for example: { "hello": "world"} ) the JSON schema for validation would be:

body: {
  type: 'object',
  properties: {
    hello: {
      type: 'string'
    }
  }
}

just like it's mentioned in the fastify docs.

However, the plugin converts the non-file fields from this:

{ "hello": "world"}

to this:

{ 
  hello: {
    fieldname: "hello",
    value: "world",
    fieldnameTruncated: false,
    valueTruncated: false,
    fields: body
  }
}

and therefore the validation schema on top (from the plugin docs) makes sense.

However, the problem is that this conversion is not mentioned anywhere in the documentation and it's just really confusing and leads to issues like this: fastify/help#525 (comment)

I would suggest to mention the conversion which would explain why is the validation schema defined like it is.

[mcollina]

Would you like to send a PR?

[radomird]

@mcollina Sure, will do that.