gh-100428: Make int documentation more accurate (#100436)

- Remove first link to lexical definition of integer literal, since it
  doesn't apply (differs in handling of leading zeros, base needs to be
  explicitly specified, unicode digits are allowed)
- Better describe handling of leading zeros, unicode digits, underscores
- Base 0 does not work exactly as like a code literal, since it allows
  Unicode digits. Link code literal to lexical definition of integer
  literal.