Add more details to behavior of allow() and valid()

### Module version

17.13.3

### What documentation problem did you notice?

Please add this clearly to the details to behavior of allow() and valid() functions such that:

allow() and valid() functions short circuits the validation, if the field value is one of the values set as parameters to the given functions.

For example, allow('', null) will not trigger additional validations in the chain if the value is 'empty string', since the given value is one of the "allowed" values.

Similarly, valid('', null) will not trigger additionally chained validations if the field value is either 'empty string' or null.