docs: Expand the "BIOS bootloader" documentation for domainCaps

Rewrite some parts for clarity, elaborate the meaning of some of the XML
attributes.  And where necessary, distinguish that we're dealing with
two different XML documents here:

  - the domainCapabilities XML, to detect the host "hypervisor"
    (QEMU/KVM) capabilities, and what libvirt knows about them.

  - the guest XML definition, i.e. what features a guest can use, based
    on the capabilities (of QEMU and libvirt and the host) reported in
    the domainCapabilities XML.

Signed-off-by: Kashyap Chamarthy <kchamart@redhat.com>
Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>

docs/formatdomaincaps.html.in

@@ -143,38 +143,51 @@
 <domainCapabilities>
 </pre>
 
-    <p>The <code>firmware</code> enum corresponds to
+    <p>The <code>firmware</code> enum corresponds to the
-      <code>firmware</code> attribute of the <code>os</code> element.
+      <code>firmware</code> attribute of the <code>os</code> element in
-      Plain presence of this enum means that libvirt is capable of so
+      the domain XML. The presence of this enum means libvirt is capable
-      called firmware auto selection. The listed values then represent
+      of the so-called firmware auto-selection feature. And the listed
-      accepted values for the domain attribute. Only values for which
+      firmware values represent the accepted input in the domain
-      there exists a firmware descriptor that matches machine type and
+      XML. Note that the <code>firmware</code> enum reports only those
-      architecture are listed, i.e. those which won't cause a failure
+      values for which a firmware "descriptor file" exists on the host.
-      on domain startup.
+      Firmware descriptor file is a small JSON document that describes
+      details about a given BIOS or UEFI binary on the host, e.g. the
+      fimware binary path, its architecture, supported machine types,
+      NVRAM template, etc. This ensures that the reported values won't
+      cause a failure on guest boot.
     </p>
 
     <p>For the <code>loader</code> element, the following can occur:</p>
 
     <dl>
       <dt><code>value</code></dt>
-      <dd>List of known loader paths. Currently this is only used
+      <dd>List of known firmware binary paths. Currently this is used
-      to advertise known locations of OVMF binaries for qemu. Binaries
+      only to advertise the known location of OVMF binaries for
-      will only be listed if they actually exist on disk.</dd>
+      QEMU. OVMF binaries will only be listed if they actually exist on
+      host.</dd>
 
       <dt><code>type</code></dt>
-      <dd>Whether loader is a typical BIOS (<code>rom</code>) or
+      <dd>Whether the boot loader is a typical BIOS (<code>rom</code>)
-      an UEFI binary (<code>pflash</code>). This refers to
+      or a UEFI firmware (<code>pflash</code>). Each <code>value</code>
-      <code>type</code> attribute of the &lt;loader/&gt;
+      sub-element under the <code>type</code> enum represents a possible
-      element.</dd>
+      value for the <code>type</code> attribute for the &lt;loader/&gt;
+      element in the domain XML. E.g. the presence
+      of <code>pfalsh</code> under the <code>type</code> enum means that
+      a domain XML can use UEFI firmware via: &lt;loader/&gt;
+      type="pflash" ...&gt;/path/to/the/firmware/binary/&lt;/loader&gt;.
+      </dd>
 
       <dt><code>readonly</code></dt>
       <dd>Options for the <code>readonly</code> attribute of the
-      &lt;loader/&gt; element.</dd>
+      &lt;loader/&gt; element in the domain XML.</dd>
 
       <dt><code>secure</code></dt>
       <dd>Options for the <code>secure</code> attribute of the
-      &lt;loader/&gt; element. Note, that <code>yes</code> is listed
+      &lt;loader/&gt; element in the domain XML. Note that the
-      only if there is a firmware that supports it.</dd>
+      value <code>yes</code> is listed only if libvirt detects a
+      firmware descriptor file that has path to an OVMF binary that
+      supports Secure boot, and lists its architecture and supported
+      machine type.</dd>
     </dl>
 
     <h3><a id="elementsCPU">CPU configuration</a></h3>