Document URL: https://access.redhat.com/documentation/en-US/Red_Hat_Enterprise_Linux/7/html/Virtualization_Deployment_and_Administration_Guide/sect-Managing_guest_virtual_machines_with_virsh-Managing_snapshots.html
There are two ways to generate disk snapshots in QEMU: internal and external. We don't encourage the creation of internal qcow2 snapshots, because they're less flexible and usually not considered very robust. In RHEV and OSP, only external snapshots are supported for example.
Unfortunately, this distinction is not very clear at the libvirt level (e.g. virsh) and to make things worse, most tools (e.g. virsh again) tend to default to internal snapshots.
Our documentation should focus on internal snapshots and discourage the creation of internal snapshots. This will help prevent customers from hitting bugs in a feature which is not robust.
As a side-note, we're trying to change the defaults in future versions of virsh and virt-manager, see bug 1402581 and bug 1403951