Attribute hints are mainly used for display purpose. For example, given an attribute
isPublic, we can declare a hint Whether the post should be visible for not logged in users,
which provides user-friendly description of the attribute meaning and can be displayed to end users.
Unlike label hint will not be generated, if its explicit declaration is omitted.
Note, in order to inherit hints defined in the parent class, a child class needs to
merge the parent hints with child hints using functions such as array_merge().
