docs: hint usage of trezor-user-env

[no changelog]

docs/core/build/emulator.md

@@ -1,5 +1,8 @@
 # Build instructions for Emulator (Unix port)
 
+> :bulb: **Hint**:
+Using emulator as described here is useful during firmware development. If you intend to use the emulator without modifying the firmware, you might be looking for [Trezor User Env](https://github.com/trezor/trezor-user-env/tree/master).
+
 First clone, initialize submodules, install Poetry and enter the Poetry shell as
 defined [here](index.md). **Do not forget you need to be in a `poetry shell`
 environment!**

docs/core/emulator/index.md

@@ -1,5 +1,8 @@
 # Emulator
 
+> :bulb: **Hint**:
+Using emulator as described here is useful during firmware development. If you intend to use the emulator without modifying the firmware, you might be looking for [Trezor User Env](https://github.com/trezor/trezor-user-env/tree/master).
+
 Emulator is a unix version of Core firmware that runs on your computer.
 
 ![emulator](emulator.jpg)