Update syntax-highlighting.adoc to be more complete when describing t…

…he "no blank lines after the title" requirement

Co-authored-by: Guillaume Grossetie <[email protected]>

docs/modules/converter/pages/syntax/syntax-highlighting.adoc

@@ -11,10 +11,13 @@ You can activate Highlight.js syntax highlighting (disabled by default) by setti
 :source-highlighter: highlight.js
 ----
 
-[NOTE]
+[IMPORTANT]
 ====
-There can be no blank lines between the title and the `source-highlighter` attribute. If you insert a blank line, the source highlighter will be ignored.
+The first empty line in the document marks the end of the header.
+In other words, if you declare the `source-highlighter` attribute after an empty line it won't activate syntax highlighting.
 
+Reference: xref:asciidoc:document:header.adoc[]
+====
 [NOTE]
 ====
 By default, we are using a prebuilt version of Highlight.js with 34 commonly used languages hosted on {url-cdnjs}[cdnjs].