CONTROLS.html: catch it up with the volume move + the bindings migration

The interactive controls page had drifted from the markdown fix-ups: PgUp/PgDn
rendered as unbound keys with no meaning (the volume keys live there now), and
the Rebinding section still promised 'an existing file is never overwritten' --
made false by the board-2 migration.  Both corrected; the page and CONTROLS.txt
now tell the same story.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

docs/CONTROLS.html

@@ -708,8 +708,10 @@
     <p class="sub">
       <code>content\bindings.txt</code> is written with the full default layout
       on first run. Change a line, restart. Delete the file to restore these
-      defaults — worth doing after an update, since an existing file is never
-      overwritten.
+      defaults. Upgrading from an older build migrates your file automatically:
+      rows you wrote and the joystick wizard's section are kept (yours beat the
+      defaults), the old stock layout is replaced, and the previous file is
+      saved as <code>bindings.old.txt</code>.
     </p>
 
     <div class="two-col">
@@ -846,8 +848,8 @@
     ];
 
     var navRows = [
-      [["Ins", "", X], ["Home", "", X], ["PgUp", "", X]],
-      [["Del", "", X], ["End", "", X], ["PgDn", "", X]],
+      [["Ins", "", X], ["Home", "", X], ["PgUp", "VOL +", F, "built-in", "Game volume up (saved between sessions)"]],
+      [["Del", "", X], ["End", "", X], ["PgDn", "VOL −", F, "built-in", "Game volume down (saved between sessions)"]],
       "grow",
       [["↑", "CENTRE", F, "0x42", "Torso centre"]],
       [["←", "LOOK", F, "0x44", "Look left"], ["↓", "BEHIND", F, "0x41", "Look behind"], ["→", "LOOK", F, "0x43", "Look right"]]