Fix docstring for Servo smoothHalt function (backport #3298)

[mergify]

Description

I was debugging something related to #3251 and noticed that the docstring for Servo::smoothHalt() was slightly out of date in explaining its return type.

Super small change.

Checklist

• Required by CI: Code is auto formatted using clang-format
• Extend the tutorials / documentation reference
• Document API changes relevant to the user in the MIGRATION.md notes
• Create tests, which fail without this PR reference
• Include a screenshot if changing a GUI
• While waiting for someone to review your request, please help review another open pull request to support the maintainers

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This is an automatic backport of pull request #3298 done by [Mergify](https://mergify.com).

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 44.12%. Comparing base (f846188) to head (ef9734f).
Report is 2 commits behind head on jazzy.

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Additional details and impacted files

@@            Coverage Diff             @@
##            jazzy    #3308      +/-   ##
==========================================
- Coverage   44.13%   44.12%   -0.00%     
==========================================
  Files         700      700              
  Lines       61669    61668       -1     
  Branches     7473     7473              
==========================================
- Hits        27209    27205       -4     
- Misses      34292    34296       +4     
+ Partials      168      167       -1     

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