Add doc about movement of things like StreamSink

[fzyzcjy]

Changes

Close #1701

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.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (202b74e) 99.27% compared to head (1357228) 99.26%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1703      +/-   ##
==========================================
- Coverage   99.27%   99.26%   -0.02%     
==========================================
  Files         354      354              
  Lines       14660    14660              
==========================================
- Hits        14554    14552       -2     
- Misses        106      108       +2     

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