docs: Document NVRAM behavior on transient domains

docs: Document NVRAM behavior on transient domains

Since 1.2.8 it's possible to use OVMF on domains. Moreover, it's
possible to have libvirt create NVRAM file per domain. Later,
when domain is undefined, the file is removed too. However,
things are a bit complicated when domain's transient. There's no
undefine to transient domains. There are two options: 1) leave
the file behind and let mgmt app remove it. 2) remove it
automatically as domain dies.
But, in some scenarios mgmt app may want to preserve the file,
copy it somewhere safe, and then copy it back when the domain is
starting again. And this wouldn't be possible with case 2). So,
even though case 1) leaves some files behind (possibly undeleted
for a long time), the files themselves are small (128K each). And
data loss is worse than full disk, isn't it?

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>