Some documentation cleanups and include more comments that have been already in code into doxygen docs

doc/doxygen/main_page.h

@@ -30,3 +30,34 @@
  * @page contrib How to contribute to lwIP
  * @verbinclude "contrib.txt"
  */
+
+/**
+ * @defgroup lwip_nosys Mainloop mode ("NO_SYS")
+ * @ingroup lwip
+ * Use this mode if you do not run an OS on your system. \#define NO_SYS to 1.
+ * Feed incoming packets to netif->input(pbuf, netif) function from mainloop,
+ * *not* *from* *interrupt* *context*. You can allocate a @ref pbuf in interrupt
+ * context and put them into a queue which is processed from mainloop.\n
+ * Call sys_check_timeouts() periodically in the mainloop.\n
+ * Porting: implement all functions in @ref sys_time and @ref sys_prot.\n
+ * You can only use @ref callbackstyle_api in this mode.\n
+ * Sample code:\n
+ * @verbinclude NO_SYS_SampleCode.c
+ */
+
+/**
+ * @defgroup lwip_os OS mode (TCPIP thread)
+ * @ingroup lwip
+ * Use this mode if you run an OS on your system. It is recommended to
+ * use an RTOS that correctly handles priority inversion and
+ * to use @ref LWIP_TCPIP_CORE_LOCKING.\n
+ * Porting: implement all functions in @ref sys_layer.\n
+ * You can use @ref callbackstyle_api together with \#define tcpip_callback,
+ * and all @ref threadsafe_api.
+ */
+
+/**
+ * @defgroup raw_api RAW API
+ * @ingroup callbackstyle_api
+ * @verbinclude "rawapi.txt"
+ */