Improve documentation of slices rules

[hankem]

The user guide's slices documentation had a comment that didn't perfectly match the code:

// checks all subpackages of 'myapp' for cycles
SlicesRuleDefinition.slices().matching("..myapp.(**)").should().notDependOnEachOther()

In addition, the involved methods could use some more JavaDoc:

• matching(String)
• notDependOnEachOther()

[codecholeric]

Thanks for taking care of this 🙂