Update Combobox Lookup Examples to use pre-defined examples only

[kwilloughby-sf]

• Since the vast majority of uses cases for Lookups are to search pre-defined options default all the examples to be pre-defined by default. It's okay to allow custom entries but we should not show it in the examples. I have never seen a situation where custom entries are permitted but it is something SLDS does allow.

• Update Base-Pre-defined Options Only (http://design-system-react-components.herokuapp.com/?path=/story/sldscombobox--base-pre-defined-options-only): Per SLDS "The up and Down arrows should not update the input value to reflect the currently selected option. The input value should only be updated upon selection." The user should always be able to type in the input and it should never become locked.

[kwilloughby-sf]

I found an instance where custom entries are permitted in Tasks (Subject). In that instance, the Listbox just disappears if nothing matches. I still feel like the majority of use cases are to search pre-defined options though.

[interactivellama]

This UX pattern is doable today, but we should update the examples to show the standard SLDS pattern for the Lookup pattern.

Tasks example is an autocomplete text field only and not object/entity selection.