[Python] Document the import-time default of PeriodicImpulse start_timestamp

[raams95]

Fixes #40564

As discussed in #40565, the default start_timestamp of PeriodicImpulse (and MatchContinuously, which passes it through) stays as is, but the docstrings didn't say that the default is evaluated once at module import time. This updates both docstrings to say so, and points to passing an explicit timestamp or using rebase to start relative to pipeline execution.

Docstring-only change, so no CHANGES.md entry.

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[github-actions]

📋 Review Summary

This PR updates the docstrings for PeriodicImpulse and MatchContinuously to clarify that the default start_timestamp (Timestamp.now()) is evaluated once at module import time rather than when the transform is instantiated or the pipeline starts. It also guides users toward passing an explicit timestamp or using rebase when relative execution timing is desired.

🔍 General Feedback

• The documentation additions are clear, accurate, and adhere to the formatting conventions and line lengths of both files.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.90%. Comparing base (76f6e6d) to head (594846d).
⚠️ Report is 178 commits behind head on master.

Additional details and impacted files

@@              Coverage Diff              @@
##             master   #40599       +/-   ##
=============================================
+ Coverage     56.16%   79.90%   +23.73%     
=============================================
  Files          1124      488      -636     
  Lines        178336    82384    -95952     
  Branches       1489        0     -1489     
=============================================
- Hits         100170    65830    -34340     
+ Misses        75645    16554    -59091     
+ Partials       2521        0     -2521     

Flag	Coverage Δ
go	?
java	?
python	79.90% <ø> (+0.34%)	⬆️

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[github-actions]

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R: @tvalentyn for label python.
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[tvalentyn]

thanks!