NAME
sh - overview of various system shells
SYNOPSIS
POSIX Shell:
sh [+-aefhikmnoprstuvx] [+-o option] ... [-c string] [arg ...]
rsh [+-aefhikmnoprstuvx] [+-o option] ... [-c string] [arg ...]
Bourne Shell:
sh [--acefhiknrstuvx ...] [arg ...]
rsh [--acefhiknrstuvx ...] [arg ...]
(and so forth) Basically, this sh(1) page tells you what the shells are and which manual page to look at in order to get the detailed information.
Perhaps on your HPUX they didn't update the man pages when/if they upgraded to 10.20?