[DOC] RDoc for additions

[BurdetteLamar]

I realized in working with BigDecimal here that the user is most like to pair the serialize/deserialize operations, so have written accordingly.

[BurdetteLamar]

@hsbt, I don't think I caused these failed checks, but please correct me if I'm wrong.

[BurdetteLamar]

@hsbt, @nurse: Will either of you be reviewing this? If not, is there someone else I should ask?

[hsbt]

@BurdetteLamar I exclude truffleruby-head from CI at #551. You can resolve CI failure with git rebase.

[BurdetteLamar]

@BurdetteLamar I exclude truffleruby-head from CI at #551. You can resolve CI failure with git rebase.

@hsbt, I was not able to do this. Can you (or someone else) resolve this?

[BurdetteLamar]

As I said earlier, I was not able to untangle this PR. If no one objects, I'll close it and start fresh with a new PR; there's no technical review so far, so I think nothing would be lost.

[BurdetteLamar]

Putting into new PR.