Updated docs for onCheck properties

[shanshin]

Resolves #213

[qwwdfsad]

It seems to be an incomplete fix -- it's easy to misunderstand this documentation and still misuse API.

I suggest throwing a configuration error if onCheck is explicitly set for projects without check task

[shanshin]

I suggest throwing a configuration error if onCheck is explicitly set for projects without check ta

For this checking with it is necessary to slightly change the API - the default value will now be "no value".

[qwwdfsad]

For this checking with it is necessary to slightly change the API - the default value will now be "no value".

It is not, because no value and false are identical semantically anyway.
Our goal here is to provide fail-fast mechanism for the common scenario where user misconfigures their project by explicitly invoking onCheck(true).

It is perfectly fine not to fail on onCheck(false) as this behaviour is indistinguishable from setting nothing at all

[shanshin]

1. as far as I know, it is not possible to add a listener to Property.set(true)
2. verify tasks true by default now
3. when checking, I will not distinguish the default true from the one set explicitly
4. if I replace the onCheck property with a function with an immediate check for the presence of the check task, then this may work unstable, because the task can be created after assigning a value (eq in afterEvaluate)

[qwwdfsad]

:okayface: