Man pages matching name tcl_createfilehandler in section 3tcl:

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