doc: Clarify path search in child_process spawn

[zobo]

child_process: clarify path search in spawn(), explain difference between unix/win when PATH is not present in env option

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 that the path search will fall back on process.env.PATH.
However 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.

References:
https://developer.apple.com/library/archive/documentation/System/Conceptual/ManPages_iPhoneOS/man3/execvp.3.html
https://linux.die.net/man/3/execvp

node/deps/uv/src/unix/process.c

Line 337 in c61870c

execvp(options->file, options->args);

node/deps/uv/src/win/process.c

Line 1017 in c61870c

path = find_path(env);