[boost] Boost.IOStreams mapped_file_sink missing flush() functionality

Hello, I wish to bring to your attention two issues regarding mapped_file_sink from Boost.IOStreams (release 1.62.0): 1. There is no member function `mapped_file_sink::flush()` which I would like to use in my program. I believe it could be implemented portably by using `msync()` for POSIX and `FlushViewOfFile()` for Windows; was the omission of this function a deliberate choice or a minor oversight? 2. The member functions `begin()` and `end()` are implemented for mapped_file_sink but are not documented! This will lead to programmers relying strictly on the docs to use `data()` and `data()+size()` instead. Please update the documentation for the next release 1.63.0.