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

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

include/ruby/internal/arithmetic/mode_t.h

@@ -23,14 +23,17 @@
 #include "ruby/internal/config.h"
 #include "ruby/internal/arithmetic/int.h"
 
+/** Converts a C's `mode_t` into an instance of ::rb_cInteger. */
 #ifndef NUM2MODET
 # define NUM2MODET RB_NUM2INT
 #endif
 
+/** Converts an instance of ::rb_cNumeric into C's `mode_t`. */
 #ifndef MODET2NUM
 # define MODET2NUM RB_INT2NUM
 #endif
 
+/** A rb_sprintf() format prefix to be used for a `mode_t` parameter. */
 #ifndef PRI_MODET_PREFIX
 # define PRI_MODET_PREFIX PRI_INT_PREFIX
 #endif