Add warnings and mitigation for QubesOS/qubes-issues#6585

See the discussion on QubesOS/qubes-posts#79.

user/how-to-guides/how-to-update.md

@@ -36,7 +36,10 @@ Fully updating your Qubes OS system means updating:
 - [templates](/doc/glossary/#template)
 - [standalones](/doc/glossary/#standalone) (if you have any)
 
-You can accomplish this using the **Qubes Update** tool.
+You can accomplish this using the **Qubes Update** tool. (**Warning:** This
+tool is currently affected by bug
+[#6585](https://github.com/QubesOS/qubes-issues/issues/6585). See below for a
+mitigation.)
 
 [![Qubes Update](/attachment/doc/r4.0-software-update.png)](/attachment/doc/r4.0-software-update.png)
 
@@ -55,15 +58,41 @@ desired items from the list and clicking "Next."
 
 <div class="alert alert-danger" role="alert">
   <i class="fa fa-exclamation-triangle"></i>
-  <b>Warning:</b> Updating with direct commands such as
+  <b>Warning:</b> Updating <em>exclusively</em> with direct commands such as
   <code>qubes-dom0-update</code>, <code>dnf update</code>, and <code>apt
   update</code> is <b>not</b> recommended, since these bypass built-in Qubes OS
-  update security measures. Instead, we strongly recommend using the <b>Qubes
+  update security measures. Instead, we strongly recommend <em>first</em> using
-  Update</b> tool or its command-line equivalents, as described below. (By
+  the <b>Qubes Update</b> tool or its command-line equivalents, as described
+  below, <em>then</em> using the direct commands for confirmation (see <a
+  href="https://github.com/QubesOS/qubes-issues/issues/6585">#6585</a> and <a
+  href="https://github.com/QubesOS/qubes-posts/pull/79">PR #79</a>). (By
   contrast, <a href="/doc/how-to-install-software/">installing</a> packages
   using direct package manager commands is fine.)
 </div>
 
+As a temporary mitigation until
+[#6585](https://github.com/QubesOS/qubes-issues/issues/6585) is fixed, the
+following update sequence is recommended (see
+[PR #79](https://github.com/QubesOS/qubes-posts/pull/79)
+for explanation and discussion):
+
+1. Update dom0 with Salt.
+2. Update dom0 by direct command.
+3. Update templates and standalones with Salt.
+4. Update templates and standalones by direct commands.
+
+Example using only the command line (all commands with `sudo` or as root):
+
+1. In dom0: `qubesctl --show-output state.sls update.qubes-dom0`
+2. In dom0: `qubes-dom0-update --clean -y`
+3. In dom0: `qubesctl --show-output --skip-dom0 --templates state.sls
+   update.qubes-vm`
+4. In dom0: `qubesctl --show-output --skip-dom0 --standalones state.sls
+   update.qubes-vm`
+5. In every Fedora template and standalone: `dnf -y --refresh upgrade`
+6. In every Debian template and standalone: `apt-get clean && apt-get -y update
+   && apt-get -y dist-upgrade && apt-get clean`
+
 ### Qubes 4.0
 
 Advanced users may wish to perform updates via the command-line interface. The