Add description for trigger crash dump

Add description for server trigger crash dump operation
into api-guide.

Change-Id: I20492dbf9eab4f7fcab0864bb250b20f98b762e4
blueprint complete-todo-in-api-concept-doc

api-guide/source/server_concepts.rst

@@ -294,6 +294,13 @@ Server actions
    Administrators may use this to evacuate servers from a host that needs to
    undergo maintenance tasks.
 
+-  **Trigger crash dump**
+
+   Trigger crash dump usually utilized by either administrator or the server's
+   owner, it will dump the memory image as dump file into the given server,
+   and then reboot the kernel again. And this feature depends on the setting
+   about the NMI in the server.
+
 Server passwords
 ~~~~~~~~~~~~~~~~