Missing information about the p:os-exec cwd option

[xatapult]

The cwd option of p:os-exec specifies the current working directory. The description leaves unspecified what this is: a URI? a OS dependent string? I think we must specify this (or at least say its implementation or OS defined).

Morgana currently (on Windows) only accepts a non-URI path, like C:/x/y/z. Making it a URI (file:/ in front) results in an error.

[ndw]

I'd be happy to make it a URI. My implementation also treats it as a OS-specific string at the moment, except for a little path-separator mangling.

[ndw]

Gerrit suggests that we document that the input is passed through p:urify() to handle the Windows/Mac/Linux/cases.