add a paragraph about escaping special chararcters (see #1509)

src/ui/pages/help.html

@@ -140,7 +140,10 @@
 							cannot exceed 192 characters and are truncated if longer. The invalid characters are:
 							<code>[</code> <code>~</code> <code>+</code> <code>\</code> <code>?</code> <code>%</code>
 							<code>*</code> <code>:</code> <code>|</code> <code>"</code> <code>&lt;</code>
-							<code>&gt;</code> and control characters from 0 to 31 in the ASCII table.
+							<code>&gt;</code> and control characters from 0 to 31 in the ASCII table. When you want
+							to customize the infobar, you can prefix the special characters <code>%</code>,
+							<code>&lt;</code>, <code>&gt;</code>, <code>{</code>, <code>}</code> and <code>|</code> with
+							<code>\</code> (e.g. <code>\&gt;</code>) to keep them in the infobar content.
 						</p>
 						<p>Template examples:</p>
 						<ul>