Man page: document `nuke` action

[r1w1s1]

Hi,

While testing mtm 1.2.1 locally, I backported the nuke action in my
SlackBuild because I found it genuinely useful during daily use and
testing with full-screen apps.

I also noticed that nuke isn’t documented in the man page yet, so I
added a small manpage patch below to document it and its default key
binding.

This is documentation-only and can be taken or dropped independently.

the mtm-1.2.1-man-nuke.patch

diff --git a/mtm.1 b/mtm.1
index 4c2c3f1..XXXXXXX 100644
--- a/mtm.1
+++ b/mtm.1
@@ -102,6 +102,11 @@ While running,
 .It Em "l"
 .Pq "the letter ell"
 Redraw the screen.
+.It Em "k"
+Clear the contents of the currently focused terminal.
+This action is sometimes referred to as
+.Dq nuke
+and is useful for forcing a full redraw of the active pane.
 .It Em "PgUp/PgDown/End"
 Scroll the terminal up/down/to the bottom.
 By default,