update docs

Author: SteveBronder

stan/math/rev/core/arena_matrix.hpp

@@ -128,6 +128,11 @@ class arena_matrix : public Eigen::Map<MatrixType> {
     Base::operator=(a);
     return *this;
   }
+  /**
+   * Forces hard copying matrices into an arena matrix 
+   * @tparam T Any type assignable to `Base`
+   * @param x the values to write to `this`
+   */
   template <typename T>
   void hard_copy(const T& x) {
     Base::operator=(x);

stan/math/rev/core/var.hpp

@@ -390,7 +390,7 @@ class var_value<T, internal::require_matrix_var_value<T>> {
     reverse_pass_callback(
         [this_vi = this->vi_, other_vi = other.vi_]() mutable {
           other_vi->adj_ += this_vi->adj_;
-          //
+          // Reset the adjoint for `this` to replicate SoA before assignment
           this_vi->adj_.setZero();
         });
   }