Improve Variadic function templates article

[ntrel]

Following on from #2204:

• Add short table of contents
• Update C++ sections for C++11
• Use sequence instead of tuple.
• Minor wording changes.
• Formatting tweaks.

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[wilzbach]

I think we just need to make the HEADERTOC generator happy.

[wilzbach]

This needs to be H2 (and consequently all H2 need to be H3) as the HEADERTOC generation will fail otherwise

[wilzbach]

H3 + LNAME2 if you want this to appear on the TOC.

[ntrel]

@wilzbach Fixed using H2, tests pass. I've not added anchors for the other headings though.

[wilzbach]

Thanks!