documentation: Mention AirlineModeChanged
documentation: Mention AirlineModeChanged

In 781c40bb3d95c4d273937f99db545e the AirlineModeChanged user-defined
autocommand has been introduced. However it was forgotten to mention
this actually in the documentation.

So here we go.

closes #1774

rchicoli/vim-airline,bling/vim-airline,chrisbra/vim-airline,yzhou61/vim-airline