Pad Start and Back ship unbound

They were mapped to the config buttons (0x37/0x36), which the 9 and 0
keys already reach from the Upper Right MFD bank. Freeing them gives
players two pad buttons to assign; bindings.txt carries the mapping as
commented example lines so the way to re-enable it is in the file.
Docs and the dist README follow.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

docs/CONTROLS.md

@@ -24,7 +24,7 @@ padaxis <src>   axis <axis> [invert] [deadzone <d>] [rate <n>]
         ______________________________________________________
        /                                                      \
       /    [Back]                                  [Start]     \
-     |     config 2                                config 1      |
+     |     unbound                                  unbound      |
      |                                                           |
      |        ,--.                                    (Y)        |
      |       ( L  )  stick X / Y                   Upper         |
@@ -51,13 +51,12 @@ padaxis <src>   axis <axis> [invert] [deadzone <d>] [rate <n>]
 | **LB** | Panic | `0x3D` |
 | **RB** | Throttle button — reverse thrust | `0x3F` |
 | D-pad | Hat: up / back / left / right (look) | `0x42` `0x41` `0x44` `0x43` |
-| **Start** | Config 1 | `0x37` |
-| **Back** | Config 2 | `0x36` |
+| **Start**, **Back** | unbound — yours to map | — |
 
-Start and Back are not extra buttons: `0x37`/`0x36` are the first two
-buttons of the Upper Right MFD's top row, which is where the config
-buttons live on the real board — so the `9` and `0` keys do the same
-thing.
+The config buttons (`0x37`/`0x36`) are the first two buttons of the
+Upper Right MFD's top row, reachable from the keyboard on `9` and `0`.
+Start and Back ship unbound so you can put anything there; the
+commented lines in `bindings.txt` show how.
 
 ---