What is a valid search query for SearchUsers/?q={search} endpoint?

I've started playing around with the BNET API and I came across this method "SearchUsers/?q={search}" under https://www.bungie.net/platform/user/help/. There are two issues with it:- 1. The official documentation of this endpoint says nothing about what some valid search criteria (i.e {search}) could be. I have worked with various APIs in the past and each company defines the search parameters differently. This is very poor documentation. 2. The documentation for this endpoint says nothing about needing the APIKey to always be added to the request header. Or is it the default? If I try a HTTP GET request on that end point, I get the "Please add valid X-API-Key header to request" error status back which means the API KEY is always required. Can someone confirm? Also, this is a PUBLIC end-point as per the documentation. Then why does it require an API key to access the end point?