Man pages matching name tcl_opentcpclient in section 3tcl:

Tcl_OpenTcpClient.3tcl.en	debian 10 / tcl8.6-doc
Tcl_OpenTcpClient.3tcl.en	debian 11 / tcl8.6-doc
Tcl_OpenTcpClient.3tcl.en	debian 12 / tcl8.6-doc