Document @ConditionBlock annotation in the guide

[Vampire]

Spock 0.7 added the @ConditionBlock annotation to support user-defined blocks with implicit annotations, but it is missing from the docs.

[johannesgottschalk]

Hi @Vampire I would like to take over this issue and add the annotation to the docs. Do I need to be assigned to this ticket or can I just go with the fix? Thanks for your support!

[leonard84]

@johannesgottschalk your comment is enough, if you have any question feel free to ask.

[AndreasTu]

@leonard84 Can you have a look at the PR, if this is enough documentation for the feature? I am also not sure if the location of the docu is a good place in the spock_primer, I would say this is an advanced feature, but I did not see another section for such things, so I grouped in to the other mentions of conditions.