document is_case_visible

Signed-off-by: Pavel Boldin <[email protected]>

docs/run_test/runbook.rst

@@ -28,6 +28,7 @@ Runbook Reference
 
    -  `variable <#variable>`__
 
+      -  `is_case_visible <#is-case-visible>`__
       -  `is_secret <#is-secret>`__
       -  `file <#file>`__
       -  `name <#name-2>`__
@@ -426,6 +427,15 @@ with value ``northeurope``.
      - name: location
        value: northeurope
 
+is_case_visible
+^^^^^^^^^^^^^^^
+
+type: bool, optional, default is False.
+
+When set to True, the value of this variable will be passed to the testcases,
+such as ``perf_nested_kvm_storage_singledisk`` which requires information
+about nested image.
+
 is_secret
 ^^^^^^^^^