Update feature_advanced_keycodes.md (#3672)

* Update feature_advanced_keycodes.md

Probably a better way to word it, but I've solved this issue on reddit several times, had to solve this for friends, ect. It's something worth being in the docs as it's a common issue, and not always intuitive. I have offered this change as I was told by several people "It was never mentioned in the docs".

* Update feature_advanced_keycodes.md

Clear up what was meant to be said.

docs/feature_advanced_keycodes.md

@@ -21,7 +21,7 @@ Additionally, if at least one right-handed modifier is specified in a Mod Tap or
 
 # Switching and Toggling Layers
 
-These functions allow you to activate layers in various ways. Note that layers are not generally independent layouts -- multiple layers can be activated at once, and it's typical for layers to use `KC_TRNS` to allow keypresses to pass through to lower layers. For a detailed explanation of layers, see [Keymap Overview](keymap.md#keymap-and-layers)
+These functions allow you to activate layers in various ways. Note that layers are not generally independent layouts -- multiple layers can be activated at once, and it's typical for layers to use `KC_TRNS` to allow keypresses to pass through to lower layers. For a detailed explanation of layers, see [Keymap Overview](keymap.md#keymap-and-layers) When using momentary layer switching with MO(), LM(), TT(), or LT(), make sure to leave the key on the above layers transparent or it may not work as intended.
 
 * `DF(layer)` - switches the default layer. The default layer is the always-active base layer that other layers stack on top of. See below for more about the default layer. This might be used to switch from QWERTY to Dvorak layout. (Note that this is a temporary switch that only persists until the keyboard loses power. To modify the default layer in a persistent way requires deeper customization, such as calling the `set_single_persistent_default_layer` function inside of [process_record_user](custom_quantum_functions.md#programming-the-behavior-of-any-keycode).)
 * `MO(layer)` - momentarily activates *layer*. As soon as you let go of the key, the layer is deactivated.