Add documentation on blocked dialog

[jecisc]

Today I needed to open a dialog that do not bloc the Pharo UI but that block the execution of the method invoking the presenter.

I found that you can use #openBlockedDialog for this. Could be nice to document it.

My usecase: I'm building a presenter to ask for a protocol name and I want a method to open the presenter and return the protocol name selected when we click on Ok.

[Ducasse]

TODO: Add in the managing window a paragraph

[Ducasse]

We can get inspiration from requestProtocolNameConfiguring: aBlock
With returning a value to show the case.