documentation element: character data between child elements not allowed

[woutdenolf]

I'm trying to use xmlschema and found a validation error which we don't get with lxml (see https://github.com/nexusformat/definitions/issues/1368)

import xmlschema
xmlschema.XMLSchema("nxdl.xsd")

Traceback (most recent call last):
  File "/home/denolf/virtualenvs/nexus/lib/python3.10/site-packages/xmlschema/validators/schemas.py", line 1197, in _parse_inclusions
    self.include_schema(location, self.base_url)
  File "/home/denolf/virtualenvs/nexus/lib/python3.10/site-packages/xmlschema/validators/schemas.py", line 1264, in include_schema
    schema = type(self)(
  File "/home/denolf/virtualenvs/nexus/lib/python3.10/site-packages/xmlschema/validators/schemas.py", line 482, in __init__
    self.parse_error(e.reason or e, elem=e.elem)
  File "/home/denolf/virtualenvs/nexus/lib/python3.10/site-packages/xmlschema/validators/xsdbase.py", line 196, in parse_error
    raise error
xmlschema.validators.exceptions.XMLSchemaParseError: character data between child elements not allowed:

Schema component:

  <xs:element xmlns:xs="http://www.w3.org/2001/XMLSchema" name="example">m^2</xs:element>

Path: /xs:schema/xs:simpleType[4]/xs:annotation/xs:documentation/xs:element[1]

Schema URL: file:///tmp/nexus_definitions/nxdlTypes.xsd

Origin URL: file:///tmp/nexus_definitions/nxdl.xsd

The issue is this

<?xml version="1.0" encoding="UTF-8"?>

<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema"
	targetNamespace="http://definition.nexusformat.org/nxdl/3.1"
	xmlns:nxdl="http://definition.nexusformat.org/nxdl/3.1"
	elementFormDefault="qualified">

	<xs:simpleType name="NX_ANGLE">
		<xs:annotation>
			<xs:documentation>
				units of angle
				<xs:element name="example">rad</xs:element>
			</xs:documentation>
		</xs:annotation>
		<xs:restriction base="xs:string" />
	</xs:simpleType>
	
</xs:schema>

The docs https://www.w3schools.com/xml/el_documentation.asp say that

 <documentation
source=URI reference
xml:lang=language>

Any well-formed XML content

</documentation> 

I'm not sure what Any well-formed XML content means but in xmlschema it means either plain text or children, not both? Can anyone provide additional context? I'm not that familiar with XMLSchema.