Mention more clearly that only a subset of features of supported HTML tags are implemented

In the doc:
https://doc.qt.io/qt-6/richtext-html-subset.html

It says that rich text support is a subset of HTML 4, and then says:

Supported Tags

The following table lists the HTML tags supported by Qt's rich text engine:

This gives an impression that limited selection of tags are supported and gives a list of what is supported. However, not all aspects of those tags are fully implemented, not even all features of attributes that are explicitly mentioned there.

For example, HTML 4 supports percentages in width, height and spacing attributes, but Qt does not (except for hr tag).