doc: add 'pci-e820-host' feature description

doc/manpages/qvm-features.rst

@@ -171,6 +171,15 @@ Some systems (Windows) works better with 'cirrus' model set here.
 
 Default: vga
 
+pci-e820-host
+^^^^^^^^^^^^^
+
+Enable e820_host option in Xen domU config if qube has any PCI device assigned.
+This is option is needed for some PCI device drivers to correctly allocate
+memory. Refer to Xen documentation for details.
+
+Default: yes if qube has any PCI device, otherwise no
+
 Authors
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