[filesystem][documentation] copy_file cannot overwrite executing binary

[boost@…]

If trying to copy a binary file over the currently executing program, then the copy_file call fails even though it has the overwrite_if_exists flag.

Examining the code, this appears to be because copy_file_api (Linux) uses popen, thus attempts to write to the same file data as distinct from creating a new file node.

The peculiar error "Text file busy: <binary>" is reported.

Given that the Windows version would not (AFAIK) be able to achieve this anyway, I would say that this behaviour is "by design" but suggest a documentation update for copy_file.