This was the result of a lot of experimentation with formats for the
Glossary in the context of building out the Payment Methods doc. In the
end, the best is to structure it just like any other doc, i.e. no
special `term:: definition` syntax, as it breaks expectations like
ability to anchor link to the terms.

It was also an excercise in developing the tersest and most digestible
format possible. What's coming together here is inspired by @evoskuil's
work at https://github.com/libbitcoin/libbitcoin/wiki/Glossary.

In any case it remains very much a work in progress, but one worth
committing at this point.

Such that both definitions and examples always end in a period.

I've been unable to find a single definition for Settle that works in
all contexts without forcing awkward usage. It's a bit unfortunate to
have to introduce the 'with object' variant here, but apparently
necessary.

Previously, Markdown-style `> ` syntax was used to set off examples (a
shortcut for Asciidoctor's `[quote]` style).  This rendered poorly,
however, with the quotes being too large and distracting from the
definition. This change switches to the (aptly named) `[example]` style,
which renders more appropriately.

See https://asciidoctor.org/docs/user-manual/#style

Previously, [example] blocks rendered with a pale yellow that generally
looks good, but is a bit too eye-catching for the purposes of this
document. Asciidoctor's [sidebar] block style was closer to the mark,
but the semantics of [example] are still desirable, as that's exactly
what these blocks represent: example usages. So this change customizes
[example] blocks to use the same background color as sidebar blocks, and
only for this doc through the private docinfo mechanism [1]. It also
customizes the coloration of `code` text, as the default coloration
there is basically the same as the background, making the two wash out.

[1]: https://asciidoctor.org/docs/user-manual/#docinfo-file