DOC 1.9: clarify behavior of //model/model/static (#921)

These changes were applied to SDFormat 1.8 in #713
but were not propagated to 1.9 after being merged
forward. This completes the merge.

Signed-off-by: Steve Peters <scpeters@openrobotics.org>

Author: scpeters

sdf/1.9/model.sdf

@@ -14,17 +14,25 @@
   <attribute name="canonical_link" type="string" default="" required="0">
     <description>
       The name of the model's canonical link, to which the model's implicit
-      coordinate frame is attached. If unset or set to an empty string,
-      the first link element listed as a child of this model is chosen
-      as the canonical link.
+      coordinate frame is attached. If unset or set to an empty string, the
+      first `/link` listed as a direct child of this model is chosen as the
+      canonical link. If the model has no direct `/link` children, it will
+      instead be attached to the first nested (or included) model's implicit
+      frame.
     </description>
   </attribute>
   <attribute name="placement_frame" type="string" default="" required="0">
     <description>The frame inside this model whose pose will be set by the pose element of the model. i.e, the pose element specifies the pose of this frame instead of the model frame.</description>
   </attribute>
 
   <element name="static" type="bool" default="false" required="0">
-    <description>If set to true, the model is immovable. Otherwise the model is simulated in the dynamics engine.</description>
+    <description>
+      If set to true, the model is immovable; i.e., a dynamics engine will not
+      update its position. This will also overwrite this model's `@canonical_link`
+      and instead attach the model's implicit frame to the world's implicit frame.
+      This holds even if this model is nested (or included) by another model
+      instead of being a direct child of `//world`.
+    </description>
   </element>
 
   <element name="self_collide" type="bool" default="false" required="0">