doc: document linux pwrite() bug

On Linux, positional writes don't work when the file is opened in
append mode. The kernel ignores the position argument and always
appends the data to the end of the file.

To quote the man page:

  POSIX requires that opening a file with the O_APPEND flag should have
  no affect on the location at which pwrite() writes data.  However, on
  Linux, if a file is opened with O_APPEND, pwrite() appends data to the
  end of the file, regardless of the value of offset.