Fix CurrentPathStack::getPath() documentation that says it has no leading / (and make it easier to actually get that?)

Problem/Motivation

I'm a bit confused by #2372507-74: Remove _system_path from $request->attributes → .4 (leading /) and the changes based on that. At least, we need to fix the documentation in CurrentPathStack::getPath() because that still says it returns it without a leading /, and given that it is 50/50 when we need a leading / it, maybe there should be an easier way to get the path without leading /?

IMHO, leading / was always a good way to differentiate between a url string (which is yet another thing, as commented above, it also includes the subfolder you're in) and the path. We need to better explain who should be using it and when, the current explanation there reads like an excuse for it's existence :)

Proposed resolution

Remaining tasks

User interface changes

API changes