Clarify semantic of top level user paths

`xrGetCurrentInteractionProfile` takes a `topLevelUserPath` argument, which is not totally clear to me how it should look like in practice. My initial assumption would be a `/user path` (6.3.1) but it also states it needs to follow the device input subpath syntax, which starts to confuse me as the semantic of 'top level' is unknown to me in this context.

Start with the specification text for xrGetCurrentInteractionProfile and section 6.3.1, focusing on the topLevelUserPath argument and the device input subpath syntax. Clarify the meaning of “top level” and show how the argument should be formed in practice. Done means the relationship between user paths and device input subpaths is unambiguous.

Written by the indexing model from the issue text.