Update callouts.qmd

Added the 'caution' callout example for completeness and explained the heading behaviour

(cherry picked from commit 5c39727)

Author: serialc

docs/authoring/callouts.qmd

@@ -31,6 +31,12 @@ Callouts provide a simple way to attract attention, for example, to this warning
 Danger, callouts will really improve your writing.
 :::
 
+::: callout-caution
+## Headings are not necessary
+
+Providing a heading is optional and the keyword (Note, Warning, Important, Tip, or Caution) will be used instead.
+:::
+
 ::: callout-tip
 ## Tip With Title