docs: Clarify combos interact with hold-tap's hold-while-undecided (#3335)

Add missing board Kconfig file for properly noting ZMK board compat and
enabling boot mode retention for the `xiao_rp2040//zmk` board variant.

app/boards/seeed/xiao_rp2040/Kconfig.xiao_rp2040

@@ -0,0 +1,10 @@
+# Copyright (c) 2026 Pete Johanson
+# SPDX-License-Identifier: MIT
+
+config BOARD_XIAO_RP2040
+    select ZMK_BOARD_COMPAT if BOARD_XIAO_RP2040_RP2040_ZMK
+    imply RETAINED_MEM if BOARD_XIAO_RP2040_RP2040_ZMK
+    imply RETENTION if BOARD_XIAO_RP2040_RP2040_ZMK
+    imply RETENTION_BOOT_MODE if BOARD_XIAO_RP2040_RP2040_ZMK
+
+

docs/docs/keymaps/behaviors/hold-tap.mdx

@@ -437,6 +437,10 @@ If enabled, the hold behavior will immediately be held on hold-tap press, and wi
 };
 ```
 
+:::warning[Interactions with combos]
+If the key position that has the hold-tap assigned is _also_ used in any combos, the `hold-while-undecided` behavior isn't immediate with key-down, but waits until all those combo timeouts have expired.
+:::
+
 #### `hold-while-undecided-linger`
 
 If your tap behavior activates the same modifier as the hold behavior, and you want to avoid a double tap when transitioning from the hold to the tap, you can use `hold-while-undecided-linger`. When enabled, the hold behavior will continue to be held until _after_ the tap behavior is released.