I, personally, find the documentation of [file] quite good.
Yes, but it doesn't mention at all that relative paths are resolved to the current working directory (or any other way the OS prefers). This is goes against an established pattern and would be surprising to most users (although it's not wrong).
I think file-help.pd should mention prominently that relative file paths are generally *not* resolved to the canvas environment and you have to use either [file which] or [dir( -> [pd control]. Hopefully, the latter can soon be replaced by something like [file resolve]: https://github.com/pure-data/pure-data/issues/1539