docs(columnDef): update name regarding uniqueness
docs(columnDef): update name regarding uniqueness

Add a warning about the name needing to be unique to
all instances of grids on a page because it can be used
as a key for retrieving custom sort algorithms, etc..

Servoy/ng-grid,dbckr/ui-grid,dbckr/ui-grid,ntmuigrid/ntmuigrid,angular-ui/ui-grid,angular-ui/ui-grid,ntmuigrid/ntmuigrid,AgDude/ng-grid,InteractiveIntelligence/ng-grid,Servoy/ng-grid,Servoy/ng-grid,angular-ui/ng-grid,angular-ui/ui-grid,AgDude/ng-grid,angular-ui/ng-grid,dbckr/ui-grid,InteractiveIntelligence/ng-grid,AgDude/ng-grid,angular-ui/ng-grid,InteractiveIntelligence/ng-grid,ntmuigrid/ntmuigrid