log4j-docgen: XSD generation doesn't allow @PluginValue annotated fields to be text content of the containing element

The [log4j-config-2.xsd](https://logging.apache.org/xml/ns/log4j-config-2.xsd) file generated by `log4j-docgen` and located at https://logging.apache.org/xml/ns/ does not allow for the `` element in `log4j2.xml` to contain a value as text content.

Using the generated XSD, this is valid:
```xml

```
but this is not valid:
```xml
value
```
However, as seen [here](https://logging.apache.org/log4j/2.x/manual/configuration.html#concise-syntax), **both** are valid forms.

In [Property.java](https://github.com/apache/logging-log4j2/blob/main/log4j-core/src/main/java/org/apache/logging/log4j/core/config/Property.java), `value` is annotated with `@PluginValue`. This is not the same as `@PluginAttribute`.

As implied in the description of [PluginValue.java](https://github.com/apache/logging-log4j2/blob/main/log4j-plugins/src/main/java/org/apache/logging/log4j/plugins/PluginValue.java), the configuration for such an annotated field does not distinguish between attribute and "value" (text content), and so XSD generation of such annotated fields should allow for both (or, ideally, exclusively one or the other).