Suggest disable html escaping in templates in templates section (#1089)

book/src/template_syntax.md

@@ -584,6 +584,11 @@ let t = RenderInPlace { s1: SectionOne { a: "a", b: "b" } };
 assert_eq!(t.render().unwrap(), "Section 1: A=a\nB=b")
 ```
 
+**Note that if your inner template** like `SectionOne` **renders HTML content, you may want to
+disable escaping** when injecting it into an outer template, e.g. `{{ s1|safe }}`.
+Otherwise it will render the HTML content literally, because
+Askama [escapes HTML variables](#html-escaping) by default.
+
 See the example
 [render in place](https://github.com/djc/askama/blob/main/testing/tests/render_in_place.rs)
 using a vector of templates in a for block.