Man pages matching name tsmimehdrfieldvalueappend in section 3ts:

TSMimeHdrFieldValueAppend.3ts.en	debian 10 / trafficserver-dev
TSMimeHdrFieldValueAppend.3ts.en	debian 11 / trafficserver-dev
TSMimeHdrFieldValueAppend.3ts.en	debian 12 / trafficserver-dev