Remove -DXT1UseAlpha from the documentation if it is deprecated or not working

[kaphula]

The documentation states:

-DXT1UseAlpha <value> Encode single-bit alpha data. Only valid when compressing to DXT1 & BC1

Yet setting this flag does nothing to transparency when BC1 format is being used? Only when you set -AlphaThreshold to, anything really, the transparency in BC1 starts to work.

If -DXT1UseAlpha <value> is broken or whatever it should be removed from the documentation so it does not waste anyone's time and confuse them.