[PD] [file]: paths not relative to patch

[Christof Ressi]

> 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