Update some setting descriptions in the graphics options. Hide "Disable Destination Alpha" from wx UI.

More than a few of these have annoyed me for a while, what with their typos and odd wording and occasional lack of important details.

Criticisms welcome. I mean, as long as it doesn't devolve into MORE bikeshedding. Even I don't enjoy that.
I think it's still supposed to have that possibility. Basically, checking that option prevents high-resolution EFB copies to be thrown away in cases where the non-cached code path would reload them as low-resolution from RAM.