[Microservices] Create additional docs for @EventPattern extras parameter

[thiagomini]

I'm submitting a...

• Regression
• Bug report
• Feature request
• Documentation issue or request (new chapter/page)
• Support request => Please do not submit support request here, instead post your question on Stack Overflow.

Current behavior

There is no documentation for the extras parameter of @EventPattern(patterns, transport, extras) microservice decorator.

Expected behavior

We should have an explanation for it. quoting Kamil's response:

the third parameter might come in handy only when creating custom strategies which is still very rare and it’s rather an advanced subject, hence no docs (it’s not a priority atm)

Minimal reproduction of the problem with instructions

N/A

What is the motivation / use case for changing the behavior?

Provide more context on how to utilize this parameter.

Environment

For Tooling issues:

• Node version: n/a

• Platform: any

Others: