docs: show blank-line rule in admonition multi-line examples

Fix the Plain admonition example so it actually renders multiple
lines, and spell out that paragraphs inside an admonition must be
separated by a blank line.

Co-authored-by: Cursor <cursoragent@cursor.com>

Author: cotti

docs/syntax/admonitions.md

@@ -4,7 +4,8 @@ Admonitions allow you to highlight important information with varying levels of
 
 ## Basic admonitions
 
-Admonitions can span multiple lines and support inline formatting.
+Admonitions can span multiple lines and support inline formatting. To split the content into separate paragraphs, separate them with a blank line — adjacent lines collapse into a single paragraph.
+
 Available admonition types include:
 
 - [Note](#note)
@@ -135,6 +136,8 @@ A plain admonition is a callout with no further styling. Useful to create a call
 ::::{tab-item} Output
 
 :::{admonition} This is my callout
+This is a plain callout.
+
 It can *span* multiple lines and supports inline formatting.
 :::
 
@@ -144,6 +147,8 @@ It can *span* multiple lines and supports inline formatting.
 
 ```markdown
 :::{admonition} This is my callout
+This is a plain callout.
+
 It can *span* multiple lines and supports inline formatting.
 :::
 ```