screensaver-proxy: Better explain why some methods don't work

Bastien Nocerarequested to merge
wip/hadess/unsupported-screensaver-method into master

Clarify that the reason why a number of org.freedesktop.ScreenSaver methods aren't implemented is because they're KDE-implementation-specific, and not part of the idle inhibition specification.

Closes: #632 (closed)

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