Vimdoc needs some way to continue long @throws lines

We got rid of pipe line continuations in vimdoc. That means currently there's no way to have an `@throws` line longer than 80 characters.

Given this vimdoc comment:

```
" @throws Ambiguous if {name} refers to multiple plugins/packages within one
" source.
```

Vimdoc currently creates an awkward line wrap in the output:

```
Throws ERROR(Ambiguous) if {name} refers to multiple plugins/packages within
one
source.
```

We discussed having the convention for continuing directive lines be a standard 4-space indent:

```
" @throws Ambiguous if {name} refers to multiple plugins/packages within one
" source.
```

Other directives like `@tagline` and `@order` will need this as well.