docs: Document TPM portion of domcaps

Surprisingly, we don't document TPM part of domain capabilities.
Fortunately, the information exposed is pretty much self
explanatory, but we should document it regardless.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Peter Krempa <pkrempa@redhat.com>

docs/formatdomaincaps.rst

@@ -494,6 +494,35 @@ instance:
 ``driverType``
    Options for the ``type`` attribute of the <filesystem><driver> element.
 
+TPM device
+^^^^^^^^^^
+
+TPM device capabilities are exposed under the ``tpm`` element. For instance:
+
+::
+
+  <domainCapabilities>
+    ...
+    <devices>
+      <tpm supported='yes'>
+        <enum name='model'>
+          <value>tpm-tis</value>
+          <value>tpm-crb</value>
+        </enum>
+        <enum name='backendModel'>
+          <value>passthrough</value>
+          <value>emulator</value>
+        </enum>
+      </tpm>
+      ...
+    </devices>
+  </domainCapabilities>
+
+``model``
+   Options for the ``model`` attribute of the ``<tpm/>`` element.
+``backendModel``
+   Options for the ``type`` attribute of the ``<tpm><backend/>`` element.
+
 Features
 ~~~~~~~~