mapped_file documentation is incomplete

`mapped_file` documentation doesn't say:
1) if destructor closes file
2) how we should deal with IO errors (`std::copy(file.data(), ...)` can't throw, I guess?)

One should read source code to get that information:
https://stackoverflow.com/questions/26608264/what-exceptions-does-boost-mapped-file-source-throw
https://stackoverflow.com/questions/7342045/are-boost-mapped-files-raii

Start with the `mapped_file` source and the two linked discussions to verify whether destruction closes the file and how I/O errors are handled during operations such as `std::copy(file.data(), ...)`. Update the mapped_file documentation to answer both questions clearly, then check the resulting text against the implementation.

Written by the indexing model from the issue text.