Small documentation changes

I'm leaving the Sized kind undocumented since it isn't fully implemented
yet.
Fixed documentation for finalize->drop change
Removing the rename warning

Now that the finalize->drop rename has happened, this hopefully
shouldn't end up changing again.
auto merge of #7549 : sfackler/rust/docs, r=msullivan

I'm leaving the Sized kind undocumented since it isn't fully implemented
yet.