fastboot: update getvar documentation.

The protocol doc states that calling `fastboot getvar` for an unknown
variable should return an empty OKAY message, but in reality modern
devices return a FAIL response in this case (tested with marlin and
shamu).

Additionally, some A/B logic in fastboot expects a FAIL response for
unknown variables in order to function properly.

This CL changes the documentation to match reality.

Bug: http://b/33756193
Test: none
Change-Id: I41f7c80585be73e60ca0ac216787596c4d60a1d5