Add description of custom resource classes

Add the description about custom resource classes and
overriding standard resource classes in the "Flavors" document.

Conflicts:
    doc/source/user/flavors.rst

Due to: The following changes are not backported.

  * I9bd80adc244c64277d2d00e7d79c3002c8f9d57e
  * Ifb99e727110c4904a85bc4a13366c2cae300b8df

Change-Id: I5b804db70d229696e7b7c5b5db16946cf1f1c49f
Closes-Bug: #1800663
(cherry picked from commit 0e718ddb7a)
(cherry picked from commit e54c5fbe2d)

doc/source/user/flavors.rst

@@ -617,6 +617,28 @@ Secure Boot
   - ``disabled`` or ``optional``: (default) Disable Secure Boot for instances
     running with this flavor.
 
+Custom resource classes and standard resource classes to override
+    Added in the 16.0.0 Pike release.
+
+    Specify custom resource classes to require or override quantity values of
+    standard resource classes.
+
+    The syntax of the extra spec is ``resources:<resource_class_name>=VALUE``
+    (``VALUE`` is integer).
+    The name of custom resource classes must start with ``CUSTOM_``.
+    Standard resource classes to override are ``VCPU``, ``MEMORY_MB`` or
+    ``DISK_GB``. In this case, you can disable scheduling based on standard
+    resource classes by setting the value to ``0``.
+
+    For example:
+
+    - resources:CUSTOM_BAREMETAL_SMALL=1
+    - resources:VCPU=0
+
+    See `Create flavors for use with the Bare Metal service
+    <https://docs.openstack.org/ironic/queens/install/configure-nova-flavors>`_
+    for more examples.
+
 Required traits
     Added in the 17.0.0 Queens release.