Man-bgproc-filter is a compiled Lisp function in `man.el'.

(Man-bgproc-filter PROCESS STRING)

Manpage background process filter.
When manpage command is run asynchronously, PROCESS is the process
object for the manpage command; when manpage command is run
synchronously, PROCESS is the name of the buffer where the manpage
command is run. Second argument STRING is the entire string of output.