Tag helper comments

A few corrections to comments in the tag helpers while I was reading through

Also to note the behaviour of boolean attributes in not as documented
```ruby
input_tag :text, :name => 'username', :required => true, :autofocus => true
# Actual output is
# => <input type="text" name="username" required="required" autofocus="autofocus" />

# Not as in document comments
# => <input type="text" name="username" required autofocus />
```

I assume the second one is the desired behaviour.