doc, inspector: note that the host is optional

Document that `node --inspect=${port}` is also a viable option.

PR-URL: https://github.com/nodejs/node/pull/12149


Reviewed-By: Eugene Ostroukhov <eostroukhov@google.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>