Improve the Descriptor HowTo Guide

[geryogam]

Documentation

PR #24901 provides the following changes to the Descriptor HowTo Guide:

• fix a typo;
• fix a Sphinx tag preventing syntax highlighting;
• fix a bug in ClassMethod;
• reuse an existing cls name in Object;
• allow keyword arguments in Type.__new__ and Object.__new__;
• allow attribute lookup from a class in Member.__get__;
• use two more super() in Object and Type for consistency.

[rhettinger]

I added the Skip Issue tag to the PR. So we can close this issue and leave the PR discussion in one place.