docs: better documentation for g_file_info_copy_into()

The documentation of g_file_info_copy_into() was misleading. The
attributes are not just copied, @dest_info is also cleared at the
beginning. So any previously set attributes in @dest_info are lost.

There was a bug in gedit about this function, where some metadata were
not saved. So it might make sense to change the implementation to not
clear @dest_info, and copy one by one the attributes from @src_info to
@dest_info.

https://bugzilla.gnome.org/show_bug.cgi?id=747927