This updates to follow the attribute template where the intro should be
first.

It should perhaps say something, but this is already covered in the
attributes chapter, and I'm not sure if anything really fits here.

I personally don't think we need to explain the grammar in English. I
understand why it could be worthwhile, but I think so many parts of the
grammar are so complex that it would be infeasible to describe it all
this way.

This moves the rule to follow the template order, rewords it a bit to
simplify, and adds an example to illustrate.

Closes rust-lang#103

This is to follow the template.

In most of the examples for these attributes, there's no lead-in, so
let's remove the lead-in here.

Let's also say "form" rather than "thing", even though "thing" does
have a certain appeal to it.

In our rule for duplicates, let's add a forward reference to the
exception to that.

Finally, we say that the attribute applied at the crate level does not
remove the crate; later, for an example, let's clarify that it only
removes the content of the crate.

Most or all of this was preexisting to this PR.