Add MG_EV_HTTP_MULTIPART_REQUEST_END

This event is sent at the end of the request

PUBLISHED_FROM=0b841a26320e3edbf73126e2ed8d286d4e5f3fa8

docs/c-api/http.h/mg_set_protocol_http_websocket.md

@@ -12,11 +12,6 @@ The user-defined event handler will receive following extra events:
 - MG_EV_HTTP_REQUEST: HTTP request has arrived. Parsed HTTP request
  is passed as
   `struct http_message` through the handler's `void *ev_data` pointer.
-- MG_EV_HTTP_MULTIPART_REQUEST: A multipart POST request has received.
-  This event is sent before body is parsed. After this, the user
-  should expect a sequence of MG_EV_HTTP_PART_BEGIN/DATA/END requests.
-  This is also the last time when headers and other request fields are
-  accessible.
 - MG_EV_HTTP_REPLY: The HTTP reply has arrived. The parsed HTTP reply is
   passed as `struct http_message` through the handler's `void *ev_data`
   pointer.
@@ -38,14 +33,27 @@ The user-defined event handler will receive following extra events:
   handshake. `ev_data` is `NULL`.
 - MG_EV_WEBSOCKET_FRAME: new WebSocket frame has arrived. `ev_data` is
   `struct websocket_message *`
-- MG_EV_HTTP_PART_BEGIN: new part of multipart message is started,
-  extra parameters are passed in mg_http_multipart_part
-- MG_EV_HTTP_PART_DATA: new portion of data from the multiparted message
-  no additional headers are available, only data and data size
-- MG_EV_HTTP_PART_END: final boundary received, analogue to maybe used to
-  find the end of packet
-  Note: Mongoose should be compiled with MG_ENABLE_HTTP_STREAMING_MULTIPART
-  to enable MG_EV_HTTP_MULTIPART_REQUEST, MG_EV_HTTP_REQUEST_END,
-  MG_EV_HTTP_REQUEST_CANCEL, MG_EV_HTTP_PART_BEGIN, MG_EV_HTTP_PART_DATA,
-  MG_EV_HTTP_PART_END constants 
+
+When compiled with MG_ENABLE_HTTP_STREAMING_MULTIPART, Mongoose parses
+multipart requests and splits them into separate events:
+- MG_EV_HTTP_MULTIPART_REQUEST: Start of the request.
+  This event is sent before body is parsed. After this, the user
+  should expect a sequence of PART_BEGIN/DATA/END requests.
+  This is also the last time when headers and other request fields are
+  accessible.
+- MG_EV_HTTP_PART_BEGIN: Start of a part of a multipart message.
+  Argument: mg_http_multipart_part with var_name and file_name set
+  (if present). No data is passed in this message.
+- MG_EV_HTTP_PART_DATA: new portion of data from the multipart message.
+  Argument: mg_http_multipart_part. var_name and file_name are preserved,
+  data is available in mg_http_multipart_part.data.
+- MG_EV_HTTP_PART_END: End of the current part. var_name, file_name are
+  the same, no data in the message. If status is 0, then the part is
+  properly terminated with a boundary, status < 0 means that connection
+  was terminated.
+- MG_EV_HTTP_MULTIPART_REQUEST_END: End of the multipart request.
+  Argument: mg_http_multipart_part, var_name and file_name are NULL,
+  status = 0 means request was properly closed, < 0 means connection
+  was terminated (note: in this case both PART_END and REQUEST_END are
+  delivered).