[doc] wip: update docstrings for core.py

[nicolocin]

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Summary

Update docstrings for pydra.engine.core. Maybe pydra.engine.state as well

Checklist

• All tests passing
• I have added tests to cover my changes
• I have updated documentation (if necessary)
• My code follows the code style of this project
  (we are using black: you can pip install pre-commit,
  run pre-commit install in the pydra directory
  and black will be run automatically with each commit)

Acknowledgment

• I acknowledge that this contribution will be available under the Apache 2 license.

[djarecka]

this might depends on the specific class. Can't be generalized 

[codecov]

Codecov Report

Merging #317 (10a43a1) into master (312cce5) will decrease coverage by 0.02%.
The diff coverage is 93.75%.

@@            Coverage Diff             @@
##           master     #317      +/-   ##
==========================================
- Coverage   82.57%   82.54%   -0.03%     
==========================================
  Files          20       20              
  Lines        3880     3891      +11     
  Branches     1059     1059              
==========================================
+ Hits         3204     3212       +8     
- Misses        484      487       +3     
  Partials      192      192              

Flag	Coverage Δ
unittests	82.47% <93.75%> (-0.03%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
pydra/engine/core.py	88.45% <93.75%> (+0.21%)	⬆️
pydra/engine/helpers.py	78.69% <0.00%> (-0.76%)	⬇️

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