Clarify preferred shortcode syntax in style guide (#16791)

* Style Guide: Clarify which shortcode syntax should be preferred
Fixes #16383

* Apply style guide review suggestions

- Remove periods from main bullet points for consistency
- Change "simple content" to "plain text content" for clarity

Co-authored-by: Cam Soper <[email protected]>

---------

Co-authored-by: claude[bot] <41898282+claude[bot]@users.noreply.github.com>
Co-authored-by: Cam Soper <[email protected]>

Author: CamSoper

STYLE-GUIDE.md

@@ -103,6 +103,22 @@ This is a useful suggestion.
 
 ---
 
+## Shortcode syntax
+
+Hugo supports two shortcode notations:
+
+- **`{{% shortcode %}}`** (percent signs) - Use for shortcodes that process Markdown content. Hugo processes these before Markdown rendering
+  - Examples: `notes`, `choosable`, `details`
+
+- **`{{< shortcode >}}`** (angle brackets) - Use for shortcodes that output pre-formatted content. Hugo processes these after Markdown rendering
+  - Examples: `cleanup`, `example`
+
+**Rule of thumb:** If the shortcode uses `markdownify` internally (check `layouts/shortcodes/`), use percent signs. Otherwise, use angle brackets.
+
+Both syntaxes work for plain text content, but use percent signs for shortcodes with nested Markdown like lists or headings.
+
+---
+
 ## Paragraphs and Line Breaks
 
 - Separate paragraphs with a blank line.