docs(@nestjs/swagger): document description support in @ApiSchema

[ccaspers]

PR Checklist

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• The commit message follows our guidelines: https://github.com/nestjs/docs.nestjs.com/blob/master/CONTRIBUTING.md

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
• Docs
• Other... Please describe:

What is the current behavior?

Issue Number: support description

What is the new behavior?

Document support for descriptions in @ApiSchema

Does this PR introduce a breaking change?

• Yes
• No

Other information

• documents the changes done here: feat: support description in @ApiSchema swagger#3198

[kamilmysliwiec]

LGTM