The term man(ual) page refers to a set of reference documents that are distributed with many software packages (Tcl and Tk in particular) which are written in the *roff (nroff/troff) markup language and a special macro package known as -man, which originated on Unix systems. *roff is a family of markup languages designed to do presentation markup in ASCII text with an eye towards typesetters (troff or plain text nroff).
The man pages are organized in sections. The Tcl man pages, for instance, are organized in section 1 for the command programs tclsh and wish, the C APIs as section 3, and the tcl commands documented in section n. When using the traditional man commands, you would say
man -s n lassign
or, depending on your system, perhaps
man n lassign
HTML versions of the man pages are also available, both online and for download.
CL thinks of man pages instead as documentation content--"That is what you need? Look in the man page for ioctl(2)" is an example of this usage.
"Manpages" also might interest.
AMG: How were the above-linked HTML manual pages produced? They appear to be automatically generated from the nroff files in the doc subdirectory of the Tcl distribution, but I don't think they were done by tools/man2html.tcl.