child_process,Windows: fix docs for `spawn({shell})`

[refack]

• Version: *
• Platform: Windows
• Subsystem: child_process

https://nodejs.org/api/child_process.html#child_process_child_process_spawn_command_args_options erronously states that cmd.exe is the default shell for windows, while infact it's process.env.ComSpec.
The same goes for:

• https://nodejs.org/api/child_process.html#child_process_child_process_exec_command_options_callback
• https://nodejs.org/api/child_process.html#child_process_child_process_spawnsync_command_args_options
• https://nodejs.org/api/child_process.html#child_process_child_process_execsync_command_options

[refack]

/cc @nodejs/platform-windows @nodejs/documentation

[tniessen]

This is one of many problems arising from setting weird environments for child processes.

Technically, the documentation is not entirely incorrect. Microsoft specifies that COMSPEC must always contain the path to cmd.exe, at least for the root environment, so as long as that is true, the documentation is correct. It is still worth changing this in the documentation as - sadly - it is not always true for child processes...

[cjihrig]

I think we should say that the value defaults to COMSPEC, and if that is not set, cmd.exe is used.

[tniessen]

Ref #14157