Issue 11434: std.file.copy doesn't preserve file attributes (eg: executable mode etc)

This is my attempt to address pull request https://github.com/D-Programming-Language/phobos/pull/1747
Having no ddoc comments on these is going to confuse people reading the docs, they won't know what `preserveAttributesDefault` defaults to. So, I'd add a `version (D_Ddoc)` that explicitly documents this, e.g.:

```D
version (D_Ddoc)
{
    /// Defaults to PreserveAttributes.yes on Windows, and the opposite on all other platforms.
    PreserveAttributes preserveAttributesDefault;
}
```