Man pages matching name tcl_getindexfromobj in section 3tcl:

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