include/ruby/internal/arithmetic/intptr_t.h: add doxygen

Must not be a bad idea to improve documents. [ci skip]

include/ruby/internal/arithmetic/intptr_t.h

@@ -29,13 +29,45 @@
 #include "ruby/internal/value.h"
 #include "ruby/internal/dllexport.h"
 
-#define rb_int_new  rb_int2inum
-#define rb_uint_new rb_uint2inum
+#define rb_int_new  rb_int2inum  /**< @alias{rb_int2inum} */
+#define rb_uint_new rb_uint2inum /**< @alias{rb_uint2inum} */
 
 RBIMPL_SYMBOL_EXPORT_BEGIN()
+
+/**
+ * Converts a C's `intptr_t` into an instance of ::rb_cInteger.
+ *
+ * @param[in]  i  Arbitrary `intptr_t` value.
+ * @return     An instance of ::rb_cInteger.
+ * @note       This function always allocates Bignums, even if the given number
+ *             is small enough to fit into a Fixnum.
+ */
 VALUE rb_int2big(intptr_t i);
+
+/**
+ * Converts a C's `intptr_t` into an instance of ::rb_cInteger.
+ *
+ * @param[in]  i  Arbitrary `intptr_t` value.
+ * @return     An instance of ::rb_cInteger.
+ */
 VALUE rb_int2inum(intptr_t i);
+
+/**
+ * Converts a C's `intptr_t` into an instance of ::rb_cInteger.
+ *
+ * @param[in]  i  Arbitrary `intptr_t` value.
+ * @return     An instance of ::rb_cInteger.
+ * @note       This function always allocates Bignums, even if the given number
+ *             is small enough to fit into a Fixnum.
+ */
 VALUE rb_uint2big(uintptr_t i);
+
+/**
+ * Converts a C's `uintptr_t` into an instance of ::rb_cInteger.
+ *
+ * @param[in]  i  Arbitrary `uintptr_t` value.
+ * @return     An instance of ::rb_cInteger.
+ */
 VALUE rb_uint2inum(uintptr_t i);
 RBIMPL_SYMBOL_EXPORT_END()