Minimaly clarifies how to use streams

[patmuk]

Changes

It was unclear to me how to use frb's Streams. This small changes clarify what a user has to do, and what happens automagically :)

Checklist

• [ ] An issue to be fixed by this PR is listed above.
• [ ] New tests are added to ensure new features are working. Please refer to this page to see how to add a test.
• [ ] ./frb_internal precommit --mode slow (or fast) is run (it internal runs code generator, does auto formatting, etc).
• [ ] If this PR adds/changes features, documentations (in the ./website folder) are updated.
• [ ] CI is passing. Please refer to this page to see how to solve a failed CI.

Remark for PR creator

• ./frb_internal --help shows utilities for development.
• If fzyzcjy does not reply for a few days, maybe he just did not see it, so please ping him.

[patmuk]

LGTM! (I guess it would be great to refine the doc further, but that's out of the scope of this PR :) )

Oh! I hold back, as I did not want to change your text too much, just changing the bare minimum. Ok, for the future I know now that I can change the doc more - feel free to revert whenever you feel that a change is just a matter of taste (should fit your taste).