doc: @subsubsection Parsing of Suboptions

[Akim Demaille]

Hi!

I believe that the commet in startup.texi is right, but not the code:

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@c Getopt and argp start at the @section level so that there's
@c enough room for their internal hierarchy (mostly a problem with
@c argp).         -Miles

@include getopt.texi
@include argp.texi

@node Suboptions, Suboptions Example, Argp, Parsing Program Arguments
@c This is a @section so that it's at the same level as getopt and argp
@subsubsection Parsing of Suboptions

Having a single level of options is sometimes not enough.  There might
be too many options which have to be available or a set of options is
[...]

@node Suboptions Example, , Suboptions, Parsing Program Arguments
@subsection Parsing of Suboptions Example
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Parsing of Suboptions should definitely be a section, not a
subsubsection.

        Akim