bpo-43047: logging.config formatters documentation update (GH-24358)

The documentation for some parts of the logging.config formatters has
fallen behind the code.  For example, the dictionary-schema section
does not list the "class" attribute, however it is discussed in the
file/ini discussion; and neither references the style argument which
has been added.

This modifies the dictionary-schema formatters documentation to list
the keys available and overall makes it clearer these are passed to
create a logging.Formatter object.

The logging.Formatter documentation describes the default values of
format/datefmt and the various formatting options.  Since we have now
more clearly described how the configuration is created via this type
of object, we remove the discussion in this document to avoid
duplication and rely on users reading the referenced logging.Formatter
documenation directly for such details.

Instead of duplicating the discussion for the two config types, the
file/ini section is modified to link back to the dictionary-schema
discussion, making it clear the same arguments are accepted.

Automerge-Triggered-By: GH:vsajip