Man pages matching name qwttextlabel in section 3:

QwtTextLabel.3.en	debian 10 / libqwt-doc
QwtTextLabel.3.en	debian 10 / libqwt5-doc
QwtTextLabel.3.en	debian 11 / libqwt-doc
QwtTextLabel.3.en	debian 12 / libqwt-doc