More security docs and other minor improvements

layouts/shortcodes/block.html

@@ -0,0 +1,5 @@
+<div class='block'>
+{{ range (after 1 (split .Inner "\n")) }}
+<p>{{ markdownify . }}</p>
+{{ end }}
+</div>

layouts/shortcodes/highlight.html

@@ -5,6 +5,6 @@
 {{ $content := .Inner }}
 {{ $in := split .Inner "\n" }}
 {{ if gt (len $in) 1 }}
-{{ $content = (delimit (after 1 $in) "\n") }}
+{{ $content = (delimit (after 2 $in) "\n") }}
 {{ end }}
 {{ transform.Highlight (htmlUnescape $content) (.Get 0) (delimit $opts ",") }}