Refactor Mongoose documentation

PUBLISHED_FROM=e9a4e5c7b4a1d03b93a2a79e29de19e60e919929

docs/c-api/http_server.h/mg_parse_multipart.md

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+---
+title: "mg_parse_multipart()"
+decl_name: "mg_parse_multipart"
+symbol_kind: "func"
+signature: |
+  size_t mg_parse_multipart(const char *buf, size_t buf_len, char *var_name,
+                            size_t var_name_len, char *file_name,
+                            size_t file_name_len, const char **chunk,
+                            size_t *chunk_len);
+---
+
+Parses the buffer `buf`, `buf_len` that contains multipart form data chunks.
+Stores the chunk name in a `var_name`, `var_name_len` buffer.
+If a chunk is an uploaded file, then `file_name`, `file_name_len` is
+filled with an uploaded file name. `chunk`, `chunk_len`
+points to the chunk data.
+
+Return: number of bytes to skip to the next chunk or 0 if there are
+        no more chunks.
+
+Usage example:
+
+```c
+   static void ev_handler(struct mg_connection *nc, int ev, void *ev_data) {
+     switch(ev) {
+       case MG_EV_HTTP_REQUEST: {
+         struct http_message *hm = (struct http_message *) ev_data;
+         char var_name[100], file_name[100];
+         const char *chunk;
+         size_t chunk_len, n1, n2;
+
+         n1 = n2 = 0;
+         while ((n2 = mg_parse_multipart(hm->body.p + n1,
+                                         hm->body.len - n1,
+                                         var_name, sizeof(var_name),
+                                         file_name, sizeof(file_name),
+                                         &chunk, &chunk_len)) > 0) {
+           printf("var: %s, file_name: %s, size: %d, chunk: [%.*s]\n",
+                  var_name, file_name, (int) chunk_len,
+                  (int) chunk_len, chunk);
+           n1 += n2;
+         }
+       }
+       break;
+``` 
+