revise warning about fallback font

docs/modules/theme/pages/fallback-font.adoc

@@ -18,12 +18,16 @@ CAUTION: Using the fallback font slows down PDF generation slightly because it h
 Its use is not recommended for large documents.
 Instead, it's best to select primary fonts that have all the characters you need.
 
-WARNING: If the PDF generator (Prawn) makes use of the fallback font to render a character, that will introduce a break opportunity, regardless of what role is set on the span of text or what the adjacent characters are (e.g., `{nbsp}`).
-When Prawn uses a fallback font, it disrupts assumptions about line wrapping.
+[WARNING]
+====
+If the PDF generator (Prawn) makes use of the fallback font to render a character, that will introduce a break opportunity, regardless of what role is set on the span of text or what characters are adjacent (e.g., `\{nbsp}`).
+When Prawn makes use of a fallback font, it disrupts assumptions about line wrapping.
 In order to assign the fallback font to the character, Prawn must create a dedicated fragment for that character.
-In other words, using the fallback font to resolve the glyph for a character creates a fragment.
+In other words, using the fallback font to resolve the glyph for a character introduces a new fragment.
 A fragment boundary always creates an artificial break opportunity in the text.
+
 The best way to ensure that the no-break boundaries are honored is to ensure that the primary font contains glyphs for all the characters in the manuscript, as well as any required characters for using a custom font with Asciidoctor PDF.
+====
 
 == Requirements