Improve docs for `single_line_if_else_max_width` & add tests

[truls-p]

Closes #5496.

I am afraid I do not have much experience with ASTs and rustfmt in general, but I have tried to understand the implications of the single_line_if_else_max_width config option by stepping through the code and coming up with some examples + tests.

Please let me know if you have any comments or I have missed something significant that should be added to Configurations.md to further clarify this config option.

[truls-p]

Thanks again for all the help, @ytmimi ! I think I have now incorporated all the requested changes.

[ytmimi]

Great! I'll look to do a follow up after work