gh-100428: Make int documentation more accurate (GH-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.
(cherry picked from commit edfbf56f4ca6588dfd20b53f534a4465e43c82bd)

Co-authored-by: Shantanu <12621235+hauntsaninja@users.noreply.github.com>