improve documentation for has_cached_role?

[jjb]

Hi - thanks for a great project!

The docs for has_cached_role? suggest that it might sometimes fail

my experiments with code similar to what's shown in the readme do not reproduce the problem

here, foo is a real role my system uses, and does_not_exist is not used and is here just a placeholder in an attempt to generate an AR relation without the roles preloaded

the results of has_cached_role? are accurate - preloading doesn't seem to be an issue

User.without_role(:does_not_exist).map{|u| u.has_cached_role?(:foo) }

maybe this behavior has been improved since the feature and docs were introduced? #369