improve docs about grouped settings
improve docs about grouped settings

See https://github.com/atom/settings-view/pull/698#issuecomment-171725215
rlugojr/atom,decaffeinate-examples/atom,helber/atom,bsmr-x-script/atom,brettle/atom,tjkr/atom,FIT-CSE2410-A-Bombs/atom,Ingramz/atom,me-benni/atom,andrewleverette/atom,AdrianVovk/substance-ide,ashneo76/atom,decaffeinate-examples/atom,gontadu/atom,gontadu/atom,rlugojr/atom,t9md/atom,bj7/atom,Austen-G/BlockBuilder,Mokolea/atom,CraZySacX/atom,stinsonga/atom,daxlab/atom,jordanbtucker/atom,decaffeinate-examples/atom,sotayamashita/atom,scippio/atom,daxlab/atom,me-benni/atom,kevinrenaers/atom,ardeshirj/atom,AdrianVovk/substance-ide,helber/atom,bsmr-x-script/atom,andrewleverette/atom,tjkr/atom,bolinfest/atom,ardeshirj/atom,brumm/atom,Austen-G/BlockBuilder,t9md/atom,sotayamashita/atom,xream/atom,hellendag/atom,Arcanemagus/atom,brettle/atom,johnhaley81/atom,atom/atom,transcranial/atom,bj7/atom,me-benni/atom,liuderchi/atom,helber/atom,PKRoma/atom,Mokolea/atom,AlexxNica/atom,jordanbtucker/atom,florianb/atom,Austen-G/BlockBuilder,brumm/atom,stinsonga/atom,Ingramz/atom,Ingramz/atom,atom/atom,florianb/atom,xream/atom,FIT-CSE2410-A-Bombs/atom,wiggzz/atom,brettle/atom,transcranial/atom,johnhaley81/atom,t9md/atom,gontadu/atom,bolinfest/atom,wiggzz/atom,kevinrenaers/atom,Arcanemagus/atom,AlexxNica/atom,Austen-G/BlockBuilder,CraZySacX/atom,johnhaley81/atom,Mokolea/atom,tjkr/atom,AdrianVovk/substance-ide,jordanbtucker/atom,Austen-G/BlockBuilder,scippio/atom,decaffeinate-examples/atom,Arcanemagus/atom,ashneo76/atom,atom/atom,FIT-CSE2410-A-Bombs/atom,transcranial/atom,florianb/atom,liuderchi/atom,PKRoma/atom,sotayamashita/atom,bolinfest/atom,rlugojr/atom,stinsonga/atom,daxlab/atom,hellendag/atom,scippio/atom,ardeshirj/atom,bj7/atom,kevinrenaers/atom,ashneo76/atom,florianb/atom,florianb/atom,CraZySacX/atom,Austen-G/BlockBuilder,liuderchi/atom,bsmr-x-script/atom,brumm/atom,hellendag/atom,stinsonga/atom,AlexxNica/atom,xream/atom,wiggzz/atom,PKRoma/atom,liuderchi/atom,andrewleverette/atom