Poor/missing documentation of Balloon parameters

[prensing]

The parameters involved in the balloon control (in VBoxBalloonCtrl) are very poorly described (section 9.22.1). I had to read the code to understand how they are being used.

BalloonLowerLimitMB - despite the confusing name, this parameter is the minimum free memory that should be maintain in the Guest. It is not a "balloon lower limit", it is a Guest free memory lower limit.

VBOXWATCHDOG_BALLOON_SAFETYMARGIN - this parameter is only visible in the daemon start up script. While I assumed this was the memory safety margin in the Guest, it is not. It is the memory safety margin the Host. The balloon daemon will not reduce the balloon in a Guest if that would cause the Host to not have enough memory.