Clarify WebSocket quickstart example

[ncoghlan]

What do these changes do?

I initially thought this example was missing some "break"
calls before realising that WebSocketResponse would be
taking care of the loop termination internally.

I suspect I may not be the only person to make that
mistake, so this proposes an inline comment in the
example pointing out what is actually going on.

Are there changes in behavior for the user?

No

Related issue number

N/A

Checklist

• [-] I think the code is well written
• [-] Unit tests for the changes exist
• Documentation reflects the changes
• [-] If you provide code modification, please add yourself to CONTRIBUTORS.txt
  • The format is <Name> <Surname>.
  • Please keep alphabetical order, the file is sorted by names.
• Add a new news fragment into the CHANGES folder
  • name it <issue_id>.<type> for example (588.bugfix)
  • if you don't have an issue_id change it to the pr id after creating the pr
  • ensure type is one of the following:
    • .feature: Signifying a new feature.
    • .bugfix: Signifying a bug fix.
    • .doc: Signifying a documentation improvement.
    • .removal: Signifying a deprecation or removal of public API.
    • .misc: A ticket has been closed, but it is not of interest to users.
  • Make sure to use full sentences with correct case and punctuation, for example: "Fix issue with non-ascii contents in doctest text files."

[codecov-io]

Codecov Report

Merging #3559 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #3559   +/-   ##
=======================================
  Coverage   97.91%   97.91%           
=======================================
  Files          43       43           
  Lines        8577     8577           
  Branches     1384     1384           
=======================================
  Hits         8398     8398           
  Misses         74       74           
  Partials      105      105

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[asvetlov]

thanks, @ncoghlan