Document and type more subprocess.* properties/methods

[ehmicky]

This PR adds better documentation and types for subprocess.stdin, subprocess.stdout, subprocess.stderr, subprocess.stdio, subprocess.all, subprocess.send(), subprocess.on('message') and subprocess.pid.

This intentionally leaves out the following subprocess.* properties/methods:

• subprocess.channel: advanced feature, not very useful (?)
• subprocess.connected, subprocess.disconnect(): IPC features which will be superseded by our upcoming IPC helper (IPC helper #900)
• subprocess.exitCode: users should use result.exitCode instead
• subprocess.signalCode: users should use result.signal instead
• subprocess[Symbol.dispose]: terminating processes with a signal is not graceful, and we should not promote it
• subprocess.killed: confusing property which does not mean what people think it does. It just means subprocess.kill() has been called, successfully or not, and with any signal (including non-termination signals). It is just meant for idempotency, i.e. is not very useful since subprocess.kill() is already idempotent.
• subprocess.unref() and subprocess.ref(): arguably useful in some edge cases? Left out for the moment, but I'm happy to document it if you think we should.
• subprocess.spawnargs, subprocess.spawnfile: can be useful for debugging, but maybe not that much. I have personally never used it, despite the thousands of tests on Execa that required some levels of debugging.

A link to the official Node.js documentation is still left, since those are still available (and typed).

[sindresorhus]

Unrelated, but would be nice to make this undefined instead of null.