Directives above `@usage` header only apply to first `@usage`

If vimdoc directives are above the first `@usage` line in a doc comment block with multiple `@usage`s, they only apply to the first `@usage`.

For instance, given:

``` VimL
""
" @dict SomeDict
" @usage {X}
" Foo
" @usage {X} {Y}
" Bar
function! x#Foo(…
```

only the first usage will get `@dict SomeDict` and show up in the docs as `SomeDict.Foo`. The second usage will show up as `x#Foo` since it's not considered a dict function.

This can be worked around by placing all directives that should apply to all usages after an `@all` directive, or by making sure they come after each `@usage` directive, but it's unintuitive and can bury directives that you'd expect to see on the first line.