doc: add rdmsr/wrmsr to the "ACRN Shell Commands" documentation

Add 'rdmsr' and 'wrmsr' to the "ACRN Shell Commands" documentation. Both are
already described via the interactive help command but not (yet) included in our
on-line documentation.

Tracked-On: #2684
Signed-off-by: Geoffroy Van Cutsem <geoffroy.vancutsem@intel.com>

Author: Geoffroy Van Cutsem

doc/user-guides/acrn-shell.rst

@@ -44,3 +44,9 @@ The ACRN hypervisor shell supports the following commands:
      - Display the CPUID leaf [subleaf], in hexadecimal
    * - reboot
      - Trigger a system reboot (immediately)
+   * - rdmsr [-p<pcpu_id>] <msr_index>
+     - Read the Model-Specific Register (MSR) at index ``msr_index`` (in
+       hexadecimal) for CPU ID ``pcpu_id``
+   * - wrmsr [-p<pcpu_id>] <msr_index> <value>
+     - Write ``value`` (in hexadecimal) to the Model-Specific Register (MSR) at
+       index ``msr_index`` (in hexadecimal) for CPU ID ``pcpu_id``