`makeRelative`: document the laws

In GitLab by @abel on Oct 10, 2022, 20:58

See https://hackage.haskell.org/package/filepath-1.4.100.0/docs/System-FilePath.html#v:makeRelative .
I would intuitively have expected `makeRelative` to be a partial inverse to ``, so that the following law holds:
```haskell
(makeRelative root path == Just rest) == (root rest == path)
```
However, the actual function is total and we have `makeRelative root path = path` whenever we should actually get `makeRelative root path = Nothing`. But also, we have e.g. `makeRelative "." path = path`.

So, what would actually the laws be that specify `makeRelative`? And, could they please be added to the documentation?

Start with the linked System.FilePath.makeRelative documentation and examine the stated examples, including makeRelative "." path and the proposed inverse law. Determine and formalize the actual laws that describe its total behavior, then add those laws and clarifying examples to the documentation.

Written by the indexing model from the issue text.