Log4J2: PatternLayout "charset" in documentation but forbidden by xml schema

## Description

I have a config file for Log4j 2.x in XML format:

```xml

```

The XML validation is telling me:
> Attribute charset is not allowed here

The documentation for Log4j 2.x is telling me that an attribut `charset` exists in `PatternLayout`: https://logging.apache.org/log4j/2.x/manual/pattern-layout.html#plugin-attr-charset

Reading the website published at https://logging.apache.org/xml/ns, I get the impression that https://logging.apache.org/xml/ns/log4j-config-2.xsd is the official "configuration file xml schema".

## Configuration

**Version:** Log4j 2.x