content: Note highlighting behavior when lang not specified

[jmooring]

See https://discourse.gohugo.io/t/bug-or-feature-language-less-code-blocks-use-default-formatting/57001.

Places to check:

• https://gohugo.io/content-management/syntax-highlighting/#lang
• https://gohugo.io/functions/transform/highlight/
• https://gohugo.io/content-management/syntax-highlighting/#guesssyntax
• https://gohugo.io/shortcodes/highlight/#lang
• https://gohugo.io/render-hooks/code-blocks/#examples (add an example that sets default lang)