Man-filter-list is a variable defined in `man.el'.
Its value is

(("sed" "" "-e '/^[-][-]*$/d'" "-e '/[789]/s///g'" "-e '/Reformatting page. Wait/d'" "-e '/Reformatting entry. Wait/d'" "-e '/^[ ]*Hewlett-Packard[ ]Company[ ]*-[ ][0-9]*[ ]-/d'" "-e '/^[ ]*Hewlett-Packard[ ]*-[ ][0-9]*[ ]-.*$/d'" "-e '/^[ ][ ]*-[ ][0-9]*[ ]-[ ]*Formatted:.*[0-9]$/d'" "-e '/^[ ]*Page[ ][0-9]*.*(printed[ ][0-9\\/]*)$/d'" "-e '/^Printed[ ][0-9].*[0-9]$/d'" "-e '/^[ ]*X[ ]Version[ ]1[01].*Release[ ][0-9]/d'" "-e '/^[A-Za-z].*Last[ ]change:/d'" "-e '/^Sun[ ]Release[ ][0-9].*[0-9]$/d'" "-e '/[ ]*Copyright [0-9]* UNIX System Laboratories, Inc.$/d'" "-e '/^[ ]*Rev\\..*Page [0-9][0-9]*$/d'") ("awk" "'\n" "BEGIN { blankline=0; anonblank=0; }\n" "/^$/ { if (anonblank==0) next; }\n" "{ anonblank=1; }\n" "/^$/ { blankline++; next; }\n" "{ if (blankline>0) { print \"\"; blankline=0; } print $0; }\n" "'"))

Original value was

Manpage cleaning filter command phrases.
This variable contains a list of the following form:

'((command-string phrase-string*)*)

Each phrase-string is concatenated onto the command-string to form a
command filter. The (standard) output (and standard error) of the Un*x
man command is piped through each command filter in the order the
commands appear in the association list. The final output is placed in
the manpage buffer.

You can customize this variable.