feat: update grpc docs

[gperdomor]

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
[x] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] Other... Please describe:

What is the current behavior?

Issue Number: nestjs/nest#4799

What is the new behavior?

Does this PR introduce a breaking change?

[ ] Yes
[ ] No

Other information