doc: fix HTTP res 'finish' description

[tadjik1]

In addition to #21047, after #20611 close event will be emitted on response object after finish.

Checklist

• documentation is changed or added
• commit message follows commit guidelines

[tadjik1]

And one thought in addition to that: before #21047 close event was the only way to handle connection terminated by client because this event was emitted only in that case.
Now code for handling these situations will look like:

res.on('close', _ => {
  if (res.finished) return;
  
  // res.finished is false, which means that connection was terminated
  // so do some cleanup
});

Probably it is worth to be mentioned somewhere in documentation.

[addaleax]

Landed in f386c7e 🎉