Refactor Mongoose documentation

PUBLISHED_FROM=e9a4e5c7b4a1d03b93a2a79e29de19e60e919929

docs/overview/event-handler.md

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+---
+title: Event handler function
+---
+
+Each connection has an event handler function associated with it. That function
+must be implemented by the user. Event handler is the key element of the Mongoose
+application, since it defines the application's behaviour. This is what an event
+handler function looks like:
+
+```c
+static void ev_handler(struct mg_connection *nc, int ev, void *ev_data) {
+  switch (ev) {
+    /* Event handler code that defines behavior of the connection */
+    ...
+  }
+}
+```
+
+- `struct mg_connection *nc`: Connection that has received an event.
+- `int ev`: Event number, defined in `mongoose.h`. For example, when data
+  arrives on an inbound connection, `ev` would be `MG_EV_RECV`.
+- `void *ev_data`: This pointer points to the event-specific data, and it has
+  a different meaning for different events. For example, for an `MG_EV_RECV` event,
+  `ev_data` is an `int *` pointer, pointing to the number of bytes received
+  from the remote peer and saved into the receive IO buffer. The exact meaning of
+  `ev_data` is described for each event. Protocol-specific events usually have
+  `ev_data` pointing to structures that hold protocol-specific information.
+
+NOTE: `struct mg_connection` has `void *user_data` which is a placeholder for
+application-specific data. Mongoose does not use that pointer. Event handler
+can store any kind of information there.