troff reads text files (written using an instream formatting language, see nroff) and formats the text for typesetting or laser printing.
The most common modern implementation is GNU groff.
The connection to Tcl is that the Tcl man pages - the only documentation that comes with the source - uses the *roff markup formatting language.
Firstly, always remember to look at other manual pages to see what sort of things belong!
Tcl manual pages start with:
.so man.macros .TH name section version Tcl "Tcl Built-In Commands" .BS .SH NAME names \- description .SH SYNOPSIS .nf summary of commands/functions .fi .BE
The name is the name of the manual page. The section is (typically) n for commands (e.g. lsearch), 3 for C API functions (e.g. Tcl_SetObjResult), or 1 for programs/shells (e.g. tclsh). The version is the version of Tcl/extension where the page last had substantive changes. The names is a comma-separated list of all the commands or functions documented on the page, and must (along with the description) be on a single line. The description is a very short description of what the commands/functions do (e.g. lsearchs' page says "See if a list contains a particular element"), and must be on a single line (along with the names). The summary of commands/functions is what it says; check some examples for how to write them.