It is very difficult to find the place in the standard that specifies the semantics of R | C where R is a range, unless you already happen to know that C is called a "range adaptor closure object".

Under the top-level entry for operator| in the library index, there ought to be a subentry that says "range adaptor closure objects, [range.adaptor.object]" or something similar.

Relatedly, "range adaptor closure object" itself should be in the main index but is not.