Update and simplify NaN-safe mode instructions

Author: devmotion

docs/src/user/advanced.md

@@ -149,27 +149,20 @@ ForwardDiff's `NaN`-safe mode by using the
 the `nansafe_mode` preference to true, for example via:
 
 ```julia
-julia> using ForwardDiff, Preferences
+julia> using Preferences
 
 julia> set_preferences!(ForwardDiff, "nansafe_mode" => true)
-```
-
-Note that Julia has to be restarted and ForwardDiff has to be reloaded after changing
-this preference.
-
-Alternatively, you can set the preference before loading ForwardDiff, for example via:
-
-```julia
-julia> using Preferences, UUIDs
-
-julia> set_preferences!(UUID("f6369f11-7733-5829-9624-2563aa707210"), "nansafe_mode" => true)
 
 julia> using ForwardDiff
 
 julia> log(ForwardDiff.Dual{:tag}(0.0, 0.0))
 Dual{:tag}(-Inf,0.0)
 ```
 
+!!! warn
+    The preference has to be set **before** loading ForwardDiff.
+    If you change it after loading ForwardDiff, you have to restart your Julia session and reload ForwardDiff for it to become active.
+
 In the future, we plan on allowing users and downstream library authors to dynamically
 enable [`NaN`-safe mode via the `AbstractConfig`
 API](https://github.com/JuliaDiff/ForwardDiff.jl/issues/181).