docs(websockets): clarify @WebSocketServer() with namespace

[chsjen492]

PR Checklist

Please check if your PR fulfills the following requirements:

• 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?

The documentation does not explain that the type of the instance injected into the @WebSocketServer() decorator changes when the namespace option is used in the @WebSocketGateway() decorator.

Issue Number: #2934

What is the new behavior?

The documentation now explains how the type of the instance injected into the @WebSocketServer() decorator changes when using the namespace option in the @WebSocketGateway() decorator.

Does this PR introduce a breaking change?

• Yes
• No

Other information

[kamilmysliwiec]

LGTM