docs(tabs): move selecting tab code down to usage
docs(tabs): move selecting tab code down to usage

add more basics to usage.

references #5819

brianespinosa/ionic2-sass,cambronjay/ionic,jarrodkoch/ionic,TillaTheHun0/ionic,dennybiasiolli/ionic,cambronjay/ionic,joshesp/ionic,alan-agius4/ionic,driftyco/ionic,janglednerves/ionic,joshesp/ionic,manucorporat/ionic,jhlee8804/ionic,alan-agius4/ionic,TillaTheHun0/ionic,jhlee8804/ionic,janglednerves/ionic,alan-agius4/ionic,jarrodkoch/ionic,janglednerves/ionic,manucorporat/ionic,janglednerves/ionic,manucorporat/ionic,AbraaoAlves/ionic,cambronjay/ionic,dennybiasiolli/ionic,jarrodkoch/ionic,dennybiasiolli/ionic,jhlee8804/ionic,driftyco/ionic,driftyco/ionic,manucorporat/ionic,manucorporat/ionic,driftyco/ionic,joshesp/ionic,janglednerves/ionic,jhlee8804/ionic,dennybiasiolli/ionic,alan-agius4/ionic,driftyco/ionic,joshesp/ionic,alan-agius4/ionic,jarrodkoch/ionic,AbraaoAlves/ionic,AbraaoAlves/ionic,cambronjay/ionic,cambronjay/ionic,TillaTheHun0/ionic,AbraaoAlves/ionic,jhlee8804/ionic,AbraaoAlves/ionic,alan-agius4/ionic,TillaTheHun0/ionic,dennybiasiolli/ionic,jarrodkoch/ionic