Add comments on trailing uint8[1] members in the "Common" structures (#1189)

Add comments to avoid abusing these members to store extra data.

core/iwasm/common/wasm_runtime_common.h

@@ -303,6 +303,11 @@ typedef struct WASMModuleCommon {
        Wasm_Module_AoT, and this structure should be treated as
        AOTModule structure. */
     uint32 module_type;
+
+    /* The following uint8[1] member is a dummy just to indicate
+       some module_type dependent members follow.
+       Typically it should be accessed by casting to the corresponding
+       actual module_type dependent structure, not via this member. */
     uint8 module_data[1];
 } WASMModuleCommon;
 
@@ -314,6 +319,11 @@ typedef struct WASMModuleInstanceCommon {
        Wasm_Module_AoT, and this structure should be treated as
        AOTModuleInstance structure. */
     uint32 module_type;
+
+    /* The following uint8[1] member is a dummy just to indicate
+       some module_type dependent members follow.
+       Typically it should be accessed by casting to the corresponding
+       actual module_type dependent structure, not via this member. */
     uint8 module_inst_data[1];
 } WASMModuleInstanceCommon;
 
@@ -377,6 +387,11 @@ typedef struct WASMRegisteredModule {
 
 typedef struct WASMMemoryInstanceCommon {
     uint32 module_type;
+
+    /* The following uint8[1] member is a dummy just to indicate
+       some module_type dependent members follow.
+       Typically it should be accessed by casting to the corresponding
+       actual module_type dependent structure, not via this member. */
     uint8 memory_inst_data[1];
 } WASMMemoryInstanceCommon;