Add docstrings to FuzzyDict methods

[swastim01]

Description

Added a proper docstring to FuzzyDict.copy explaining its return value and behavior.
This improves the API documentation and helps users understand how to use this method correctly.

Contribution to #3392
Fixes #4845

Type of change

• Documentation update
  Update in CHANGELOG.md:

- Added docstring to `FuzzyDict.copy` explaining its return value and behavior. ([#5242](https://github.com/pybamm-team/PyBaMM/pull/5242))

Important checks:

Please confirm the following before marking the PR as ready for review:
[x] No style issues: nox -s pre-commit
[x] All tests pass: nox -s tests
[x] The documentation builds: nox -s doctests
[x] Code is commented for hard-to-understand areas
[x] Tests added that prove fix is effective or that feature works

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.78%. Comparing base (f27f7ee) to head (2ec5fd3).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #5242   +/-   ##
========================================
  Coverage    98.78%   98.78%           
========================================
  Files          321      321           
  Lines        27643    27643           
========================================
  Hits         27306    27306           
  Misses         337      337           

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