provide a setter for doclet scope (#574)

We previously set the doclet's scope by adding a
`@scope` tag, which, in turn, was used to update
the scope. Since this tag isn't defined or used in
any other context, it could cause "not a known
tag" errors.
alphabetize the tag definitions
add `@also` to tag definitions (#574)

This tag is used internally. It previously caused
"not a known tag" errors when
tags.allowUnknownTags was set to false.