Clarify website docs on vendor flag or default to auto

[peternewman]

https://www.claudiokuenzler.com/nagios-plugins/check_esxi_hardware.php
"If no hw/vendor type was given, unknown will be used (like auto)"
In fact auto autodetects/selects a vendor name if it can, which unknown doesn't.

Also is there any reason unknown is the default rather than auto? Changing this would allow it to return more info by default (especially on HP), then you could switch to unknown if it was breaking on your hardware.