contains the type of the text portion.
Text - string content
TextField - TextField content
TextContent - text content - supplied via the interface XContentEnumerationAccess
Footnote - a footnote or an endnote
ControlCharacter - a control character
ReferenceMark - a reference mark
DocumentIndexMark - a document index mark
Bookmark - a bookmark
Redline - a redline portion which is a result of the change tracking feature.
Ruby - a ruby attribute which is used in Asian text.
The text portions for reference marks and document index marks are generated twice for
each mark since they may include string content.