Update "Resource URL" / "Directory" docs
Based on comments in https://civicrm.stackexchange.com/questions/19495/set-civicrm-files/19498?noredirect=1#comment21951_19498 , it sounds like https://wiki.civicrm.org/confluence/display/CRMDOC/Resource+URLs probably needs to be revisited in light of how 4.7.x has updated path handling.
- Before 4.7, the UI always displayed absolute URLs/paths, so that's you had to put them into the GUI. But internally, the old versions would sometimes relativize the URLs/paths, but it wasn't reliable or consistent enough to explain/present.
- 4.7 introduced the variables/prefixes which (imho) made it a bit more transparent. Now-a-days, if you actually use an absolute path, the system-status check will actually display a suggestion to convert to path-variable.