Mark head follow as experimental

It works but is only lightly tested, and what's left is mostly tuning its
settings. Frametop Input Settings now says so under the Head follow switch,
and the README, design notes, and button action name say it too.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

pointer/helper/ft-pointer.cpp

@@ -86,7 +86,7 @@
 // doesn't jump the cursor to free space or swap in the laser-catching dot, which made
 // SteamVR's resize snap back.
 //
-// Head follow (off by default; POINTER_FOLLOW=1, or the relay's "follow toggle"): the cursor
+// Head follow (experimental, off by default; POINTER_FOLLOW=1, or the relay's "follow toggle"): the cursor
 // is carried by a reference direction, where the head faced when it last settled, and turns
 // with it, keeping its offset (mouse movement changes the offset, up to POINTER_FOLLOW_REACH,
 // 70 deg, so the cursor can sit in a corner of the view). While the head stays within