style(validators): add an arbitrary jsdoc comment to fix closure compiler builds

Closure compiler complains that the factory function for these validator
directives do not have the parameter documented in the preceding jsdoc
comment. These comments are actually describing the directives and not
their factory functions and so have `@param` tags that do not match the
factory function signature.

This change adds a simple empty(ish) jsdoc comment between the
directive's jsdoc comment and the factory function, which silences
the error from the closure compiler. Luckily the ngdoc processing
does not require these jsdoc comments to be attached to any code
items, so that is not affected by this change.