memory-mapped file documentation heavily implies that we cannot use a Filesystem path

[Austin Hicks <camlorn38@…>]

According to some Stack Overflow answers and the headers for iostreams, it appears that it is possible to create a mapped_file_source, mapped_file_sync, etc. using a filesystem v3 path. The documentation makes it appear that we can only use std::string, thus not properly allowing for Unicode in paths. If this is not documented because it's not complete, then I'll file something about iostreams needing support for unicode paths, but I've found some Stack Overflow answers saying it works as far back as 1.48. I'm assuming the docs just need updating.