Reword docs to not use "keys"

[martinklepsch]

this is a case of maybe unfortunate wording... when the docs talk about "applying to a key" it actually means "applying to a key's value"). Will have to reword this, since I now see this isn't immediately obvious. All of the bundled validators in this lib currently only deal with validating the values, not the keys directly.

Just logging this here from #3 so it's not forgotten :)