Update Combobox Lookup Examples to use pre-defined examples only

• 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.

Start with the Base-Pre-defined Options Only Storybook example linked in the issue and inspect the related Combobox examples. Compare their behavior with the stated SLDS rules: examples should use predefined options, arrow navigation should not change the input until selection, and typing should remain possible. Done means the examples demonstrate those behaviors without showing custom-entry use cases.