btrfs hosts: qcow2/raw disks are created without nocow

### Description

On btrfs hosts (Fedora/openSUSE default) Lima creates disk images with CoW. A qcow2 on a datacow btrfs fragments heavily over time.
There is also no mention of it in the docs.

Nocow has trade-offs: on btrfs it implies nodatasum (no checksums/self-heal), and a nocow file can't be reflinked to/from a regular one - so flagging only the disk file would break the reflink copy from the download cache and `limactl clone`. Flagging the directories (instance dir + download cache) at creation keeps reflinks working, since nocow <-> nocow reflink is fine.

Proposal:
1. Docs/FAQ note for btrfs hosts: `chattr +C` on `${LIMA_HOME}` and the download cache before first use, with the trade-offs.
2. Maybe: set `nocow` on newly created instance/cache dirs when on btrfs, as libvirt [>=6.6.0 does for new storage pools](https://libvirt.org/formatstorage.html#features) (it's opt out there) - or maybe opt-in flag?

Apologies if this has already been discussed.

Start by reviewing the documented FAQ area and the creation flow for the instance directory and download cache. Confirm the btrfs nocow trade-offs described in the issue, then resolve whether the behavior is documentation-only or an opt-in/opt-out feature. Done means the chosen policy is documented and tested without breaking reflinks or clone behavior.

Written by the indexing model from the issue text.