Add explanation about strings vs. prose

[alerque]

...and add (self promotional) suggestion for decasify plugin to handle prose. I initially landed on this plugin when I was looking for a way to fix header casing in Markdown, and it wasn't clear to me that it wasn't for the kind of title casing I was looking for. Since I've now written a tool that handles the other kind, I hope this clarification will be useful to others landing here too.