Fix documentation about struct RData's data field

Also adds a static assertion to ensure the documented behavior stays
true, namely that the data field is at the same position in the RData
and RTypedData structs.

Author: etiennebarrie

include/ruby/internal/core/rdata.h

@@ -143,8 +143,8 @@ struct RData {
     RUBY_DATA_FUNC dfree;
 
     /** Pointer to the actual C level struct that you want to wrap.
-      * This is in between dmark and dfree to allow DATA_PTR to continue
-      * to work for both RData and non-embedded RTypedData.
+      * This is after dmark and dfree to allow DATA_PTR to continue to work for
+      * both RData and non-embedded RTypedData.
       */
     void *data;
 };

include/ruby/internal/core/rtypeddata.h

@@ -37,6 +37,7 @@
 #include "ruby/internal/dllexport.h"
 #include "ruby/internal/error.h"
 #include "ruby/internal/fl_type.h"
+#include "ruby/internal/static_assert.h"
 #include "ruby/internal/stdbool.h"
 #include "ruby/internal/value_type.h"
 
@@ -374,6 +375,8 @@ struct RTypedData {
     void *data;
 };
 
+RBIMPL_STATIC_ASSERT(data_in_rtypeddata, offsetof(struct RData, data) == offsetof(struct RTypedData, data));
+
 RBIMPL_SYMBOL_EXPORT_BEGIN()
 RBIMPL_ATTR_NONNULL((3))
 /**