Remove documentation for readmore, logged_in and is_admin from node.html.twig

Problem/Motivation

These variables are set in user_template_preprocess_default_variables_alter() and don't need to be duplicated in the node.html.twig documentation, which is approximately a metre long.

Steps to reproduce

Proposed resolution

Remaining tasks

Remove the same documentation lines from duplicates of node.html.twig which is included in various core themes.

User interface changes

API changes

Data model changes

Release notes snippet