Manual: "Running External Programs" does not explain how to change working directory

[jiahao]

I was surprised that this wasn't documented in the manual. Would be useful to have a section about Cmd objects and how you can customize them with the dir kwarg to set the working directory, and more generally the manipulation of working environments with setenv() and withenv().

All of these are explained in the standard library, but it would be convenient to have a description in the manual, with suitable cross-references to the existing standard library documentation.

A simple example

Cmd(`pwd`, dir="..")

[nalimilan]

(I think needs-docs is for PRs which cannot be merged until docs have been added; docs would probably be more appropriate.)

[akaysh]

@jiahao This is pending from quite some time now. I think it would be appropriate to add a subsection called "Cmd" Object under the "Running External Programs" section with references across std-lib docs.
Any suggestions?