viewport: Remove mention of wl_surface.attach x/y

This paragraph contains an incomplete definition of how
wl_surface.attach x/y arguments functions, and makes no reference to the
similar wl_surface.offset.

The paragraph states that there is no effect on the behavior of
wl_surface.attach. Rather than elaborating on its definition and adding
wl_surface.offset, remove the paragraph and let their definition be up
to wl_surface itself.

Signed-off-by: Kenny Levinsen <kl@kl.wtf>