doc: describe some features and structures.

src/suricata.h

@@ -38,6 +38,21 @@
  *  - \ref sigstate
  *  - \ref threshold
  *
+ * \section archi Architecture
+ *
+ * \subsection datastruct Data structures
+ *
+ * Regarding matching, there is three main data structures which are:
+ *  - ::Packet: Data relative to an individual packet with information about
+ *  linked structure such as the ::Flow the ::Packet belongs to.
+ *  - ::Flow: Information about a flow for example a TCP session
+ *  - ::StreamMsg: structure containing the reassembled data
+ *
+ *  \subsection runmode Running mode
+ *
+ *  Suricata is multithreaded and running modes define how the different
+ *  threads are working together. You can see util-runmodes.c for example
+ *  of running mode.
  */
 
 /**