Wflow_sbm figure leakage term and ksathorfrac documentation

[verseve]

Issue addressed

Fixes #297, #279

Explanation

Added leakage term (and some minor improvements) to the wflow_sbm figure of the documentation.
Document external input parameter ksathorfrac.

Checklist

• Branch is up to date with master
• Tests & pre-commit hooks pass
• Updated documentation if needed
• Updated changelog.md if needed

[verseve]

@JoostBuitink : what did you use for the format changes, is that JuliaFormatter (with default settings)? That would be the preferred formatting style.

[JoostBuitink]

@verseve Oops, that was VSCode formatting the code for me (I assumed it was following JuliaFormatter, but that was not the case). The formatting changes should now be reverted, as I ran JuliaFormatter on this file.