Remove mentions of Bignum in time.c

Move section on internal details out of the class-level documentation,
since users do not need to know that.  Update it to use
Integer(T_BIGNUM) instead of Bignum.

Fixes [Misc #17580]

time.c

@@ -5080,7 +5080,7 @@ time_mdump(VALUE time)
          * Append extended year distance from 1900..(1900+0xffff).  In
          * each cases, there is no sign as the value is positive.  The
          * format is length (marshaled long) + little endian packed
-         * binary (like as Fixnum and Bignum).
+         * binary (like as Integer).
          */
         size_t ysize = rb_absint_size(year_extend, NULL);
         char *p, *const buf_year_extend = buf + base_dump_size;
@@ -5589,6 +5589,16 @@ rb_time_zone_abbreviation(VALUE zone, VALUE time)
     return rb_obj_as_string(abbr);
 }
 
+/*  Internal Details:
+ *
+ *  Since Ruby 1.9.2, Time implementation uses a signed 63 bit integer or
+ *  Integer(T_BIGNUM), Rational.
+ *  The integer is a number of nanoseconds since the _Epoch_ which can
+ *  represent 1823-11-12 to 2116-02-20.
+ *  When Integer(T_BIGNUM) or Rational is used (before 1823, after 2116, under
+ *  nanosecond), Time works slower than when integer is used.
+ */
+
 /*
  *  Time is an abstraction of dates and times. Time is stored internally as
  *  the number of seconds with subsecond since the _Epoch_,
@@ -5605,13 +5615,6 @@ rb_time_zone_abbreviation(VALUE zone, VALUE time)
  *  (Since Ruby 2.7.0, Time#inspect shows subsecond but
  *  Time#to_s still doesn't show subsecond.)
  *
- *  Since Ruby 1.9.2, Time implementation uses a signed 63 bit integer,
- *  Bignum or Rational.
- *  The integer is a number of nanoseconds since the _Epoch_ which can
- *  represent 1823-11-12 to 2116-02-20.
- *  When Bignum or Rational is used (before 1823, after 2116, under
- *  nanosecond), Time works slower as when integer is used.
- *
  *  = Examples
  *
  *  All of these examples were done using the EST timezone which is GMT-5.