Document tomllib's limits & extensions

[encukou]

Documentation

tomllib places some limits on the values it handles (like limiting integers to int_max_str_digits) and preserves some info that TOML standard doesn't guarantee (like dict order).

Document those.

See discussion: https://discuss.python.org/t/clarify-whether-tomllib-preserves-table-key-order-from-input/108698

Linked PRs

• gh-156414: Document tomllib's limits & "extensions" #156415
• [3.14] gh-156414: Document tomllib's limits & "extensions" (GH-156415) #156833
• [3.13] gh-156414: Document tomllib's limits & "extensions" (GH-156415) #156834
• [3.15] gh-156414: Document tomllib's limits & "extensions" (GH-156415) #156835