Vimdoc needs some way to continue long @throws lines

[Soares]

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.