[DOC] Tweak "Time Internal Representation"

- Remove `T_BIGNUM` word
- Mark up function names and type names as code
- Mark up struct member names as variables

timev.rb

@@ -47,37 +47,36 @@
 #
 # == \Time Internal Representation
 #
-# Time implementation uses a signed 63 bit integer, Integer(T_BIGNUM), or
+# Time implementation uses a signed 63 bit integer, Integer, or
 # Rational.
 # It is a number of nanoseconds since the _Epoch_.
 # The signed 63 bit integer can represent 1823-11-12 to 2116-02-20.
 # When Integer or Rational is used (before 1823, after 2116, under
 # nanosecond), Time works slower than when the signed 63 bit integer is used.
 #
-# Ruby uses the C function "localtime" and "gmtime" to map between the number
+# Ruby uses the C function +localtime+ and +gmtime+ to map between the number
 # and 6-tuple (year,month,day,hour,minute,second).
-# "localtime" is used for local time and "gmtime" is used for UTC.
+# +localtime+ is used for local time and "gmtime" is used for UTC.
 #
-# Integer(T_BIGNUM) and Rational has no range limit,
-# but the localtime and gmtime has range limits
-# due to the C types "time_t" and "struct tm".
+# Integer and Rational has no range limit, but the localtime and
+# gmtime has range limits due to the C types +time_t+ and <tt>struct tm</tt>.
 # If that limit is exceeded, Ruby extrapolates the localtime function.
 #
 # The Time class always uses the Gregorian calendar.
 # I.e. the proleptic Gregorian calendar is used.
 # Other calendars, such as Julian calendar, are not supported.
 #
-# "time_t" can represent 1901-12-14 to 2038-01-19 if it is 32 bit signed integer,
+# +time_t+ can represent 1901-12-14 to 2038-01-19 if it is 32 bit signed integer,
 # -292277022657-01-27 to 292277026596-12-05 if it is 64 bit signed integer.
-# However "localtime" on some platforms doesn't supports negative time_t (before 1970).
+# However +localtime+ on some platforms doesn't supports negative +time_t+ (before 1970).
 #
-# "struct tm" has tm_year member to represent years.
-# (tm_year = 0 means the year 1900.)
-# It is defined as int in the C standard.
-# tm_year can represent between -2147481748 to 2147485547 if int is 32 bit.
+# <tt>struct tm</tt> has _tm_year_ member to represent years.
+# (<tt>tm_year = 0</tt> means the year 1900.)
+# It is defined as +int+ in the C standard.
+# _tm_year_ can represent between -2147481748 to 2147485547 if +int+ is 32 bit.
 #
-# Ruby supports leap seconds as far as if the C function "localtime" and
-# "gmtime" supports it.
+# Ruby supports leap seconds as far as if the C function +localtime+ and
+# +gmtime+ supports it.
 # They use the tz database in most Unix systems.
 # The tz database has timezones which supports leap seconds.
 # For example, "Asia/Tokyo" doesn't support leap seconds but