The TCL build scripts install the man pages automatically based on the .SH NAME section. This was changed before 8.6.0 b2. Before, the tclvars(n) man page was installed. Now it isn't.. The fix would be to add tclvars back to the list.
'\" Note: do not modify the .SH NAME line immediately below!
-tclvars \- Variables used by Tcl
+argc, argv, argv0, auto_path, env, errorCode, errorInfo, tcl_interactive, tcl_library, tcl_nonwordchars, tcl_patchLevel, tcl_pkgPath, tcl_platform, tcl_precision, tcl_rcFileName, tcl_traceCompile, tcl_traceEval, tcl_wordchars, tcl_version \- Variables used by Tcl