Clarify error message styling

[CobaltCause]

In this section, it's hinted that error messages should not assume they are the beginning of the sentence/message/log line/whatever and always start with a lowercase letter (unless the first word happens to be an initialism/acronym). I think it would be a good idea to codify this in a paragraph explaining why they're written this way which would explicitly make this a requirement to be considered to be following the guidelines.

[Enet4]

Might be worth coordinating the discussion with the knowledge from the error handling working group (see for example rust-lang/project-error-handling#27).

[CobaltCause]

Oh cool, didn't know this existed 😅

[matklad]

Note that this is currently codified in stdlib docs: https://doc.rust-lang.org/stable/std/error/trait.Error.html

Error messages are typically concise lowercase sentences without trailing punctuation:

[CobaltCause]

lol I must have missed that, thanks for pointing it out