[PATCH proxmox-backup 1/1] faq: add warning about using apt upgrade

[Nicolas Frey <n.frey@proxmox.com>]

as it has increasingly come up in enterprise support and is generally an
easy foot gun for people coming from other Debian-based distros.

Signed-off-by: Nicolas Frey <n.frey@proxmox.com>
---

Notes:
    A similar patch that adds this warning was recently applied to PVE,
    see commit 86bddb2e3d39 in `pve-docs` for more context

 docs/faq.rst | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/docs/faq.rst b/docs/faq.rst
index 4ae4573c5..e9ba32609 100644
--- a/docs/faq.rst
+++ b/docs/faq.rst
@@ -48,6 +48,12 @@ through the CLI with:
   apt update
   apt full-upgrade
 
+.. warning:: As Proxmox Backup Server uses a rolling release model on
+   top of Debian, performing an `apt upgrade` may result in a partially
+   upgraded or broken package state. This is because `apt full-upgrade`
+   can also remove existing packages to satisfy dependencies if needed,
+   while `apt upgrade` cannot.
+
 .. note:: Always ensure you correctly setup the
    :ref:`package repositories <sysadmin_package_repositories>` and only
    continue with the actual upgrade if `apt update` did not hit any error.
-- 
2.47.3