Previously `(2..).cover?("2"..)` was false, but
`(..2).cover?(.."2")` was true. This changes it so both are false,
treating beginless ranges the same as endless ranges in regards to
type checks.
This also adds documentation to #cover? to describe behavior with
beginless and endless ranges, testing each documentation example,
which is how this bug was found.
Fixes [Bug #18155]