Improve docstring for AutoTapir

Author: willtebbutt

src/dense.jl

@@ -287,7 +287,13 @@ Defined by [ADTypes.jl](https://github.com/SciML/ADTypes.jl).
 
 # Fields
 
-  - `safe_mode::Bool`: whether to run additional checks to catch errors early. On by default. Turn off to maximise performance if your code runs correctly.
+  - `safe_mode::Bool`: whether to run additional checks to catch errors early. While this is
+    on by default to ensure that users are aware of this option, you should generally turn
+    it off for actual use, as it has substantial performance implications.
+    If you encounter a problem with using Tapir (it fails to differentiate a function, or
+    something truly nasty like a segfault occurs), then you should try switching `safe_mode`
+    on and look at what happens. Often errors are caught earlier and the error messages are
+    more useful.
 """
 Base.@kwdef struct AutoTapir <: AbstractADType
     safe_mode::Bool = true