Add keycodes for swap-hands feature.

docs/keycodes.md

@@ -390,3 +390,15 @@ This is a reference only. Each group of keys links to the page documenting their
 |------------|-------|-------------------------------------------------|
 |`UNICODE(n)`|`UC(n)`|Send Unicode character `n`                       |
 |`X(n)`      |       |Send Unicode character `n` via a different method|
+
+## [Swap Hands](feature_swap_hands.md)
+
+|Key        |Description                                                              |
+|-----------|-------------------------------------------------------------------------|
+|`SH_T(key)`|Sends `key` with a tap; momentary swap when held.                        |
+|`SW_ON`    |Turns on swapping and leaves it on.                                      |
+|`SW_OFF`   |Turn off swapping and leaves it off. Good for returning to a known state.|
+|`SH_MON`   |Swaps hands when pressed, returns to normal when released (momentary).   |
+|`SH_MOFF`  |Momentarily turns off swap.                                              |
+|`SH_TG`    |Toggles swap on and off with every key press.                            |
+|`SH_TT`    |Toggles with a tap; momentary when held.                                 |