Man pages matching name sd_is_fifo in section 3:

sd_is_fifo.3.en	debian 10 / libelogind-dev-doc
sd_is_fifo.3.en	debian 10 / libsystemd-dev
sd_is_fifo.3.en	debian 11 / libelogind-dev-doc
sd_is_fifo.3.en	debian 11 / libsystemd-dev
sd_is_fifo.3.en	debian 12 / libelogind-dev-doc
sd_is_fifo.3.en	debian 12 / libsystemd-dev