Docs base solver

[Jiabin-Wang]

Description

This is the practice of contribute to the PyBaMM (creating PR and updating docstring). I edited docstring in function solve and step in pybamm/solvers/base_solver.py.
These have been mentioned in the #3392 (API Docs - Small fixes suitable for first time contributors)

Type of change

I have made changes following the suggestion in the #3392 API Docs - Small fixes suitable for first time contributors: pybamm.BaseSolver.solve and pybamm.BaseSolver.step

Key checklist:

• No style issues: $ pre-commit run (or $ nox -s pre-commit) (see CONTRIBUTING.md for how to set this up to run automatically when committing locally, in just two lines of code)
• All tests pass: $ python run-tests.py --all (or $ nox -s tests)
• The documentation builds: $ python run-tests.py --doctest (or $ nox -s doctests)

You can run integration tests, unit tests, and doctests together at once, using $ python run-tests.py --quick (or $ nox -s quick).

Further checks:

• Code is commented, particularly in hard-to-understand areas
• Tests added that prove fix is effective or that feature works

[Jiabin-Wang]

As part of the preparation for the April 2024 training

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.58%. Comparing base (fe4ac31) to head (bac9294).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #3967   +/-   ##
========================================
  Coverage    99.58%   99.58%           
========================================
  Files          257      257           
  Lines        21251    21251           
========================================
  Hits         21163    21163           
  Misses          88       88           

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