python: handle unterminated strings from doc-text

python: handle unterminated strings from doc-text

doc-text sends an unterminated string to the callback for doc:content.
Python doesn't handle this well.

So create a kludge:  If the key ends " unterminated", then use ->num2 as
a max length of ->str

Signed-off-by: NeilBrown <neil@brown.name>