Clarify difference between kl_divergence and convex_kl_divergence

[zer-art]

Description

This PR addresses issue #1513 by clarifying the documentation for kl_divergence and convex_kl_divergence.

The previous documentation did not explicitly state the mathematical difference between the two implementations, nor did it explain the specific use case for the convex variant.

Changes made:

• Updated kl_divergence docstring to identify it as the standard definition.
• Updated convex_kl_divergence docstring to:
  • Explicitly mention the added term: sum(exp(log_predictions)) - sum(targets).
  • Clarify that this is the "Generalized KL Divergence" intended for unnormalized distributions.
  • Improved phrasing for readability and elegance.

Fixes #1513

Type of change

• [x] Documentation update (no code logic change)