Inconsistent interline space in multitable

[Akim Demaille]

Hi all,

Bison's documentation includes a table about the short options, long options, 
and corresponding directives.  Some long options do not have a short option 
and/or a directive.

The source looks like this:

> @multitable {@option{--force-define=@var{name}[=@var{value}]}} {@option{-F 
> @var{name}[=@var{value}]}} {@code{%nondeterministic-parser}}
> @headitem Long Option @tab Short Option @tab Bison Directive
> 
> @item @option{--color[=@var{when}]}
> @tab
> @tab
> 
> @item @option{--debug}
> @tab @option{-t}
> @tab @code{%debug}
> 
> @item @option{--define=@var{name}[=@var{value}]}
> @tab @option{-D @var{name}[=@var{value}]}
> @tab @code{%define @var{name} [@var{value}]}

As you can see, the space between the lines is inconsistent:

The result is comparable in Info:

> This is cross.info, produced by makeinfo version 6.6 from cross.texi.
> 
> Long Option                     Short Option        Bison Directive
>                                                     
> ---------------------------------------------------------------------------------
> '--color[=WHEN]'                                    
> '--debug'                       '-t'                '%debug'
>                                                     
> '--define=NAME[=VALUE]'         '-D NAME[=VALUE]'   '%define NAME [VALUE]'
>                                                     
> '--defines[=FILE]'              '-d'                '%defines ["FILE"]'
>                                                     
> '--feature[=FEATURES]'          '-f [FEATURES]'     
> '--file-prefix=PREFIX'          '-b PREFIX'         '%file-prefix "PREFIX"'
>                                                     
> '--force-define=NAME[=VALUE]'   '-F NAME[=VALUE]'   '%define NAME [VALUE]'
>                                                     
> '--graph[=FILE]'                '-g [FILE]'         
> '--help'                        '-h'                
> '--language=LANGUAGE'           '-L LANGUAGE'       '%language "LANGUAGE"'
>                                                     
> '--locations'                                       '%locations'

HTML is correct.

I believe the input is correct, but maybe I should use some explicit means to 
denote "empty content"?

Attached is a reduced version of the problem.

Cheers!

cross.texi
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