Allow docvim blocks to appear inside function bodies

Allow docvim blocks to appear inside function bodies

For example, I just had a case where I had an option defined/used inside
a function:

  function s:foo()
    ""
    " @option g:SomeOption boolean 1
    "
    " This option does some thing.
    let l:dispatch=get(g:, 'SomeOption', 1)

    " [code that uses the option here...]
  endfunction

Note that @option is a bit different from @mapping and @command. The
latter generally will have one authoritative place where they are
defined and where their documentation can logically/obviously go. An
@option on the other hand, may be used in multiple places.

So it may make sense to either just document these elsewhere (like up in
the @plugin area), or get into the habit of making a centralized place
where option default values are defined, so that they can be documented
independently of their usage sites.