inspector: document bad usage for --inspect-port

Document --inspect-port, and fix the reporting for when it is misused.

The option requires an argument, but when the argument was omitted, the
error message incorrectly reported --inspect-port as being bad, as if
was not supported at all:

    % node --inspect-port
    node: bad option: --inspect-port
    % node --none-such
    node: bad option: --none-such

It is now correctly reported as requiring an argument:

    % ./node --inspect-port
    ./node: --inspect-port requires an argument

PR-URL: https://github.com/nodejs/node/pull/12581


Reviewed-By: James M Snell <jasnell@gmail.com>