The logical structure can contain a copy of text which is drawn
on the page.
This is held in the logical structure because it is to get hold
or and also because it may be subtly different.
For example the text "ll" might be represented using a single
ligature in the page content,.
whereas the text in the logical structure might use a double
One is optimized for display. The other is optimized for
In cases where text on the page is changed the actual text may
no longer be correct.
In particular, if text is redacted, you do not want a copy of
the text to be maintained.
This function updates the actual text in the structure.