libinput.accelProfile: improve docs & new default

The link to some (of course non-existing, i.e. freedesktop) “libinput”
documentation is replaced by a piece of the API documentation.

The default is changed since the documentation suggests `adaptive`
should be it.
https://wayland.freedesktop.org/libinput/doc/latest/group__config.html#gad63796972347f318b180e322e35cee79

Also fix a missing string conversion for `scrollButton`.