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.
The UI's job is not to educate users about terms that are used on the forums, though. Rather keep the UI clean than add stuff for no reason (and maybe instead make it a habit to just say External FB instead of XFB :p)