feat: support description in @ApiSchema

[ccaspers]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/nestjs/nest/blob/master/CONTRIBUTING.md
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

Related PR to update docs can be found here:

• docs(@nestjs/swagger): document description support in @ApiSchema docs.nestjs.com#3157

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Other... Please describe:

What is the current behavior?

Issue Number: 2671

What is the new behavior?

When using the @ApiSchema decorator, it is now possible to specify the model's description.

@ApiSchema({ description: "Model Description"})
class Model {
   // ...
} 

When rendering the openapi spec, the description will be added to document like this:

{
  // ...
  "schemas": {
    "Model": {
      "type": "object",
      "description": "Model Description"
      //...
    }
  }
  // ...
}

Does this PR introduce a breaking change?

• Yes
• No

Other information

[kamilmysliwiec]

LGTM