Man pages matching name curlinfo_starttransfer_time in section 3:

CURLINFO_STARTTRANSFER_TIME.3.en	debian 10 / libcurl4-doc
CURLINFO_STARTTRANSFER_TIME.3.en	debian 11 / libcurl4-doc
CURLINFO_STARTTRANSFER_TIME.3.en	debian 12 / libcurl4-doc