Documentation for the XsltArgumentList.AddExtensionObject does dont specify the API of the object properly

### Type of issue

Missing information

### Description

The extension object is called from the XSLT processor XslCompiledTransform, but the documentation misses important rules and behaviours.

It doesn't explain/define arguments well, it's assumed the example is sufficient.
Can complex types be passed in and out, Trees, or collections of nodes, or just scalars - what are the types ?
Critically It doesn't define and guarantee thread expectations. Will the XSLT processor call more than one function at once in different threads ?

The documentation of this feature is very poor.

### Page URL

https://learn.microsoft.com/en-us/dotnet/api/system.xml.xsl.xsltargumentlist.addextensionobject?view=net-9.0#system-xml-xsl-xsltargumentlist-addextensionobject(system-string-system-object)

### Content source URL

https://github.com/dotnet/dotnet-api-docs/blob/main/xml/System.Xml.Xsl/XsltArgumentList.xml

### Document Version Independent Id

86f62bc7-ba63-b623-9a7f-3f9e0934c012

### Platform Id

3ae03f1e-9811-d3f2-542e-f06be48f48c5

### Article author

@dotnet-bot