console_codes.4: \e[21m is now underline

Since 65d9982d7e523a1a8e7c9af012da0d166f72fc56 (4.17), it follows
xterm rather than common sense and consistency, being the only
command 1..9 where N+20 doesn't undo what N did.  As libvte
0.51.90 got changed the same way, this behaviour will probably
stay.

Signed-off-by: Adam Borowski <kilobyte@angband.pl>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>

man4/console_codes.4

@@ -258,7 +258,7 @@ The toggle meta flag
 causes the high bit of a byte to be toggled
 before the mapping table translation is done.
 T}
-21	set normal intensity (ECMA-48 says "doubly underlined")
+21	set underline (<4.17 and many other terminals set normal intensity)
 22	set normal intensity
 24	underline off
 25	blink off