Update docs to define what is meant by location identifier.

Add a description of what "location identifier" means to our documentation. Detail and clarify in the description what the identifier represents and how it relates, or lack thereof, to the other object IDs and its intended use in the library.

For a start, "location identifier refers to the ID of the object under which an object should be created/opened or the starting point of the search for things like H5Gget_info_by_idx."