doc: clarify path search in child_process.spawn

The documentation about command lookup could be more clear and note
differences between Windows and Linux/OSX.
Current text gives the impression that if one passes `options.env`
without `PATH`, the path search will fall back on `process.env.PATH`.
In reality, passing environment without `PATH` to `execvp` causes it
to look for the binary only in `/usr/bin:/bin`.
Also Windows behaves different and more in line with the current
documentation text.

PR-URL: #41418
Reviewed-By: Richard Lau <rlau@redhat.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

doc/api/child_process.md

@@ -36,8 +36,11 @@ identical to the behavior of pipes in the shell. Use the `{ stdio: 'ignore' }`
 option if the output will not be consumed.
 
 The command lookup is performed using the `options.env.PATH` environment
-variable if it is in the `options` object. Otherwise, `process.env.PATH` is
-used.
+variable if `env` is in the `options` object. Otherwise, `process.env.PATH` is
+used. If `options.env` is set without `PATH`, lookup on Unix is performed
+on a default search path search of `/usr/bin:/bin` (see your operating system's
+manual for execvpe/execvp), on Windows the current processes environment
+variable `PATH` is used.
 
 On Windows, environment variables are case-insensitive. Node.js
 lexicographically sorts the `env` keys and uses the first one that