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

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

include/ruby/internal/arithmetic/pid_t.h

@@ -23,14 +23,17 @@
 #include "ruby/internal/config.h"
 #include "ruby/internal/arithmetic/long.h"
 
+/** Converts a C's `pid_t` into an instance of ::rb_cInteger. */
 #ifndef PIDT2NUM
 # define PIDT2NUM RB_LONG2NUM
 #endif
 
+/** Converts an instance of ::rb_cNumeric into C's `pid_t`. */
 #ifndef NUM2PIDT
 # define NUM2PIDT RB_NUM2LONG
 #endif
 
+/** A rb_sprintf() format prefix to be used for a `pid_t` parameter. */
 #ifndef PRI_PIDT_PREFIX
 # define PRI_PIDT_PREFIX PRI_LONG_PREFIX
 #endif