Add a note about "Manual" payment method in Commerce

Problem/Motivation

Would be great to inform users on the module page that this might not be needed for many cases any more and manual payment method is now part of Drupal Commerce: https://www.drupal.org/project/commerce/issues/2828525

It can be added as payment method here: /admin/commerce/config/payment-gateways/add

I think that should be added at the top of the module page instead of the older information.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes