Address code review comments

Address some comments that were left in code review. This is done
separately to avoid sending the entire chain through Gerrit again.

The changes made were:

  * Add a comment describing a method of the `SelectorVisitor`

  * Removed an unnecessary `protected` from `Linter`

  * Changed parameter name in `Linter#add_lint` so it wouldn't shadow a
    method

  * Removed dead `description` method from `CapitalizationInSelector`

Change-Id: I75edfcfddc703e419dd9f9e0bbd2b274954dad7e
Reviewed-on: https://gerrit.causes.com/28845
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane@causes.com>