Backout f12266b3294785ab3927fbc85443227d43e9fe9c(Minor clean up the description of Observer interface).
Backout f12266b3294785ab3927fbc85443227d43e9fe9c(Minor clean up the description of Observer interface).

The old commit has some point which should be fixed.

saneyuki/observer-js,saneyuki/observer-js