Unify style in config support info

[EliahKagan]

This builds on b31d6b7 (#1531) by adjusting the capitalization and quoting style of string arguments in progress.rs that help document the status of what configuration options are and are not planned.

• Since I believe these strings are not usually rendered as Markdown, and most places that code formatting in Markdown would be used were already written with single quotes, this applies that quoting style consistently.
• This applies initial capitalization, which was sometimes but not always done. It does not apply a consistent style for when a string will end in a ., which seems already to differ deliberately for reasons of clarity. That can be adjusted later.
• A small amount of minor rewording for clarity is also included.

[EliahKagan]

I've prefixed the commit message title with doc: as suggested in #1531 (review). However, your wording there suggests that the effect is not certain. If you prefer not to deal with that at the same time as the release-related complexities discussed in comments at f218578, then I can amend this to remove the doc: prefix. My guess is that it's fine and you may want it, though, so I'll leave it in place unless you request a change.

[Byron]

Thanks a lot for this improvement, it's much appreciated.

Let's try doc:, it should work.