Add an ability to customize the standard docs header for setters

[Veetaha]

The initial use case for this was reported in Discord.

We need some more configurability for the standard header added by bon to the docs on setters:

Maybe something like

#[builder(default = ..., setters(doc(default(skip))))]

and if you still want to show something different in the Default clause, this syntax could be used:

#[builder(default = ..., setters(doc(default = "string explaining the default value")))]

We could also support setters(doc(default(skip))) at the top level via on(pattern, ...).

Maybe there is a more general and flexible attribute syntax that could be used here? I'd be happy to find a better design!