There are a couple of ways you could do this to create that excel style type dropdown with the function name along with a short description.
The easiest way would be, is to build a small custom suggestion box using plain HTML, CSS & Javascript. You could position it right under the input so that it shows matching items as you are typing. Then each item can display the name on one side and the description on the other and also then you can add simple keyboard navigation to make it feel like excel.
You could create an object and store functionnames along with their text in it - SUM: 'Add all numbers in a range of cells'Then when user types in the input the it would check the text by matching it with Tips object you created, if it will find a match the tooltip text will be updated. The tooltip will act like a hidden div, only visible when a match is found.
Another option would be, is to use an existing autocomplete/tooltip library like Tippy.js or a lightweight autocomplete plugin and customize it to show both text and description together.