Clarify SSZ default values

[lightclient]

I initially set out to clarify the default value of Vector, but felt like this definition would be much clearer in a table format versus a sentence.

[protolambda]

Looks good, but could use a few minor changes. Also, this PR would be nice to merge into v.0.8.3, so I would change the target base before merging.

[protolambda]

@c-o-l-o-r Can you please rebase this PR to branch v08x, and change the PR target to that too? It will get into a release quicker that way, we generally like cosmetic changes to not get delayed to a major release for no reason :).

[lightclient]

Thanks @protolambda! I just rebased against v08x.

[protolambda]

Also, we generally sign commits in the specs repository @c-o-l-o-r (with the exception for commits signed through github, i.e. using the web-interface for merges).
It would be nice if you can squash the commits (since it's a single table) and then sign it.
I understand that it's all a bit much for this PR, but future PRs will be much easier when that's familiar.

[lightclient]

@protolambda no worries! Sorry for so much back-and-forth -- I will squash into one signed commit!

[lightclient]

Okay great, I have signed my commit. 😄 Let me know if there is anything else I should do!

[protolambda]

Looks good. Not entirely sure about the syntax for Union: type_0 or Union(type_0). But that's something to revisit when unions are actually used in practice, the idea is there.