Fix docstrings for HamIsing and HamRG classes

[Parth-Garg2629]

Description

This PR fixes a minor documentation bug in the HamIsing and HamRG classes where the init docstrings were incorrectly copied from the HamHeisenberg base class.

Changes made:

• Updated the [init] docstring in [HamIsing] to state "Initialize Ising Hamiltonian." instead of "Initialize XXZ Heisenberg Hamiltonian."
• Updated the [init] docstring in [HamRG] to state "Initialize Richardson-Gaudin Hamiltonian." instead of "Initialize XXZ Heisenberg Hamiltonian."
• Expanded the class docstring for the base [HamHeisenberg] to include the mathematical formulation of the XXZ Hamiltonian and explicitly mention its relation to the [HamIsing] and [HamRG] subclasses for better clarity.

Type of change

• Documentation update

How Has This Been Tested?

This is a documentation-only update. No code logic was modified. Existing tests can be verified using pytest moha/test/test_Heisenberg.py.