Arena extens/contraction callbacks documentation does not clearly describe when callbacks could be called

[thejayps]

Raise #213 (comment)

Also see #224: what does "And check for other stuff (e.g. format methods) where we should mention that." mean?

[rptb1]

Also see #224: what does "And check for other stuff (e.g. format methods) where we should mention that." mean?

Format methods are an example of a callback where there are restrictions because they're called from signal/exception handlers. I meant that we should check that we have documented these restrictions consistently, and possibly only once (rule.general.once).