[Help] verifyexists implicitly causes checks to be ignored

`[p]helpset verifyexists t` enables a message such as `Help topic for pinh not found.` However, it also allows users to run `[p]help` on commands which cannot be used in the current context (due to user or bot permissions). This second behavior is _also_ found in `[p]helpset verifychecks f`. However, `verifychecks` also hides/shows commands blocked by checks in help commands that list possible commands.

While `verifychecks` is `True`, I believe the bot *should not* send help to the user. What it should do in this case, I do not know. The docstring of `verifyexists` should also be updated, as it currently describes what I believe is the job of `verifychecks`.
> This allows the bot to respond indicating the existence of a specific help topic even if the user can't use it.