client: fix invalid doc command for WL_MARSHAL_FLAG_DESTROY

Fixes the following warning:

src/wayland-client-core.h:125: warning: Found non-existing group 'wl_proxy' for the command '@ingroup', ignoring command

"\memberof" cannot be used here because it only works on functions. The docs for "\memberof" say that "\relates" works in a similar way.

While at it, use a "" command instead of a "@" command for consistency with the rest of the file.