Man pages matching name knet_handle_pmtud_getfreq in section 3:

knet_handle_pmtud_getfreq.3.en	debian 10 / libknet-doc
knet_handle_pmtud_getfreq.3.en	debian 11 / libknet-doc
knet_handle_pmtud_getfreq.3.en	debian 12 / libknet-doc