Improve_quality_fixes: missing docstrings, and raised exceptions missing from docstrings PART 1

[clmould]

Description

Some more improve_quality_fixes - currently focussing on missing docstrings, and raised exceptions missing from docstrings

Having to leave some:
Missing ProcessValueError in peak_b_field_at_pf_coil - from the way this error is currently written, I can't determine concisely why it is being raised (#4467 should make the ProcessValueError clearer, then this can be documented in the docstring)

Missing BadParameter in process_cli - if I included this in the docstring, it would appear in the process --help menu

Leaving scan.py alone due to scan pr #4482

This PR has worked on:
examples/
process/core/io/vary_run
process/core/solver
process/core (not in any sub folder)
process/models/
tests/
tracking/

Checklist

I confirm that I have completed the following checks:

• My changes follow the PROCESS style guide
• I have justified any large differences in the regression tests caused by this pull request in the comments.
• I have added new tests where appropriate for the changes I have made.
• If I have had to change any existing unit or integration tests, I have justified this change in the pull request comments.
• If I have made documentation changes, I have checked they render correctly.
• I have added documentation for my change, if appropriate.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 93.93939% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 48.76%. Comparing base (83d9f63) to head (8a22619).
⚠️ Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
process/models/tfcoil/superconducting.py	88.88%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4462      +/-   ##
==========================================
+ Coverage   48.67%   48.76%   +0.08%     
==========================================
  Files         151      151              
  Lines       29734    29782      +48     
==========================================
+ Hits        14474    14524      +50     
+ Misses      15260    15258       -2     

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