Improve safe mode messaging

[willtebbutt]

Addresses #168 . As discussed in SciML/ADTypes.jl#68, safe_mode is part of the public interface of ADTypes as it is a documented field of the AutoTapir type. This means that we cannot change it without making a breaking change to ADTypes. Since we can't do this, it seems preferable to simply clarify the nature of safe mode / the safety_on kwarg, then to rename it and have multiple names floating around.

The purpose of this PR, therefore, is to improve the documentation around safe_mode / safety_on, to ensure that users are aware of what it does.

[willtebbutt]

Closed in favour of improving #175

[github-actions]

Performance Ratio:
Warning: results are very approximate!

┌────────────────────────────┬────────┬─────────┬─────────────┬─────────┐
│                      Label │  Tapir │  Zygote │ ReverseDiff │  Enzyme │
│                     String │ String │  String │      String │  String │
├────────────────────────────┼────────┼─────────┼─────────────┼─────────┤
│                        sum │   49.1 │   0.458 │        3.19 │   0.681 │
│                       _sum │   6.79 │   488.0 │        27.1 │   0.121 │
│                   kron_sum │   87.8 │    3.55 │       216.0 │    25.8 │
│              kron_view_sum │   97.3 │    11.6 │       256.0 │    8.49 │
│      naive_map_sin_cos_exp │   4.12 │ missing │        8.86 │    2.78 │
│            map_sin_cos_exp │   4.76 │    1.77 │        7.48 │    3.38 │
│      broadcast_sin_cos_exp │   4.39 │     2.6 │        1.67 │    2.84 │
│                 simple_mlp │   8.89 │    3.09 │        11.4 │    3.16 │
│                     gp_lml │   15.8 │    4.92 │     missing │ missing │
│ turing_broadcast_benchmark │   8.52 │ missing │        25.7 │ missing │
└────────────────────────────┴────────┴─────────┴─────────────┴─────────┘