Clarify that attach --duration keeps tracking in the background

[swaroski]

Summary

• memray attach -o ... --duration N exits as soon as the tracker is injected; the target process keeps recording for
  N seconds. Documented, but has surprised multiple users (attach ignores --duration #701, memray attach with --duration exits immediately instead of tracking for the given time #831).
• Print a notice on exit pointing at memray detach for early stop.
• Call out the background behavior explicitly in docs/attach.rst.

Test plan

• memray attach -o out.bin --duration 10 <pid> prints the new notice and exits.
• memray detach <pid> still stops tracking before duration elapses.
• Existing tests/unit/test_attach.py passes.

Closes #831

[godlygeek]

This looks basically good to me. I have 2 small changes I'd like to request.

However, on top of those changes, this branch should only have the one commit on it (it shouldn't have the change from #907 in it) and it needs a Signed-off-by line with your real name in the commit message.

If you amend your commit message to have the right signoff, with:

git commit --amend -C HEAD -m "Signed-off-by: Suyog Bhise <bhise.suyog@gmail.com>"
git push --force-with-lease

then I can drop the other commit and make the other small changes and land this.

[godlygeek]

This one still needs the Signed-off-by: line updated to your real name, @swaroski

[codecov-commenter]

Codecov Report

❌ Patch coverage is 50.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 92.37%. Comparing base (796647f) to head (fbc6d21).

Files with missing lines	Patch %	Lines
src/memray/commands/attach.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #909      +/-   ##
==========================================
- Coverage   92.44%   92.37%   -0.07%     
==========================================
  Files          99       99              
  Lines       11786    11788       +2     
  Branches      428      429       +1     
==========================================
- Hits        10895    10889       -6     
- Misses        891      899       +8     

Flag	Coverage Δ
cpp	92.37% <50.00%> (-0.07%)	⬇️
python_and_cython	92.37% <50.00%> (-0.07%)	⬇️

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[godlygeek]

LGTM, thanks @swaroski!