Fix incorrect template declaration of lower_hull_points_2()/upper_hull_points_2() in Convex_hull_2 docs

[prajakta128]

Summary

Fixes an inconsistency in the documentation of CGAL::lower_hull_points_2() and CGAL::upper_hull_points_2() in the Convex_hull_2 package. This is a documentation-only change with no impact on compiled code or the public API.

Problem

In Convex_hull_2/doc/Convex_hull_2/CGAL/convex_hull_2.h, both functions were declared with a template parameter list that did not match the symbols used in the declaration body:

template <class InputIterator, class OutputIterator>
OutputIterator
lower_hull_points_2(InputIterator first, InputIterator beyond,
                    OutputIterator result,
                    const Traits& ch_traits = Default_traits );

Two problems here:

1. Traits is used as a type in const Traits& ch_traits, but Traits is not declared in the template parameter list (class Traits is missing).
2. = Default_traits is given as a default value, but Default_traits is not defined anywhere in the codebase.

The same issue was present for upper_hull_points_2().

Root cause

Comparing against the real implementation in Convex_hull_2/include/CGAL/convex_hull_2.h, these functions actually have two separate overloads:

// Overload 1: explicit traits class
template <class InputIterator, class OutputIterator, class Traits>
OutputIterator
lower_hull_points_2(InputIterator first, InputIterator last,
                    OutputIterator result, const Traits& ch_traits);

// Overload 2: traits deduced from the point's kernel
template <class ForwardIterator, class OutputIterator>
OutputIterator
lower_hull_points_2(ForwardIterator first, ForwardIterator last,
                    OutputIterator result);

The documentation had incorrectly merged these two distinct overloads into a single, malformed declaration instead of documenting them separately — unlike convex_hull_2() earlier in the same file, which correctly documents both overloads independently.

Fix

Split the doc declaration of lower_hull_points_2() and upper_hull_points_2() into two properly-templated declarations each, mirroring the existing correct pattern used for convex_hull_2():

• One overload takes an explicit Traits parameter (template <class InputIterator, class OutputIterator, class Traits>).
• One overload omits it and documents that the traits/kernel is deduced via std::iterator_traits and CGAL::Kernel_traits (template <class InputIterator, class OutputIterator>).

Verification

• Confirmed by cross-referencin Convex_hull_2/include/CGAL/convex_hull_2.h (actual implementation against the doc stub — the two-overload structure matches exactly.
• grep -n "Default_traits\|class Traits" on the modified file no shows class Traits only inside correctly-formed template paramete lists, and no remaining reference to the undefined Default_traits.
• This is a doc-only file consumed by Doxygen; it does not affect compilation of the library or examples.

Type of change

• Documentation fix
• Bug fix (code)
• New feature

Fixes #9574