fix bug with side modules macro and add more comments

sdk/side_module_api.h

@@ -44,7 +44,7 @@ char * module_name ## _call(const char *ptr, int length) { \
     \
     unsigned int result_size = 0; \
     for (int i = 0; i < 4; ++i) { \
-        result_size = result_size | ((unsigned int)result[i] << 8*i); \
+        result_size = result_size | (module_name ## _load(result + i) << 8*i); \
     } \
     \
     char *result_out = malloc(result_size + 1); \

sdk/syscalls_stubs.c

@@ -1,5 +1,11 @@
 #include <stdio.h>
 
+/// Some syscalls could be as the result of importing files like stdio.h that in its turn imports standart I/O
+/// descriptors like stdin/stderr/stdout. Each of them is a structure contains some callbacks (e.g.
+/// https://github.com/CraneStation/wasi-libc/blob/9bb4cc5c411af8453cbef69c137a4c2337714d89/libc-top-half/musl/src/stdio/stdin.c#L11)
+/// These stubs could provide default implementations for syscalls. And please include this file, if you are sure
+/// that these syscalls couldn't be used directly.
+
 size_t __stdio_write(FILE *f, const unsigned char *buf, size_t len) {
     return 1;
 }