Mention the scope, headers, axis and abbr attributes of tables in tagging best practices

[bertfrees]

The examples that the best practices document currently contains don't need these attributes, but the attributes are essential for the accessibility of more complex tables, so it is important to make people aware of them.

scope andheaders are used to associate the right header information with each table cell.

axis is used to provide additional information about a table header, which could be queried by the reader or could be used to render or browse the table in certain ways.

abbr is used to provide a short version of a table header, which is useful when the user agent wants to render it repeatedly. (See also #23.)

More information:

• https://www.w3.org/TR/REC-html40/struct/tables.html#h-11.4.2
• https://developer.mozilla.org/en-US/docs/Learn/HTML/Tables/Advanced#tables_for_visually_impaired_users