How to resolve "prog" in PATH?

The documentation in `spawn.mli` says:

> [prog] is not searched in [PATH]. It is up to the caller to do the path resolution before calling this function.

However, it does not explain how the caller is expected to do that.

There are perhaps many ways to achieve in OCaml, nevertheless I propose that the documentation briefly mentions which method the library author(s) had in mind, and/or which method they are actually using when faced with that task.

Even better would be an actual helper function to resolve that, e.g.

```ocaml
spawn ... ~prog:(resolve_path prog_name) ...
```

The request points to spawn.mli; start by reading the existing [prog] documentation and the surrounding spawn API. Done means documenting the intended PATH-resolution method, or clarifying whether a helper is in scope; no test file is named in the issue.

Written by the indexing model from the issue text.