KVM: Documentation: Update description of KVM_{GET,CLEAR}_DIRTY_LOG

Update various words, including the wrong parameter name and the vague
description of the usage of "slot" field.

Signed-off-by: Zenghui Yu <yuzenghui@huawei.com>
Message-Id: <20201208043439.895-1-yuzenghui@huawei.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>