Update documentation

docs/en/docs/syntax/overview.md

@@ -5,6 +5,7 @@
 - `<% %>`: Inline tag, you can write Rust code inside this tag
 - `<%= %>`: Evaluate the Rust expression and outputs the value into the template (HTML escaped)
 - `<%- %>`: Evaluate the Rust expression and outputs the unescaped value into the template
+- `<%+ %>`: Evaluate the Rust expression producing a `TemplateOnce` value, and render that value into the template
 - `<%# %>`: Comment tag
 - `<%%`: Outputs a literal '<%'
 

docs/en/docs/syntax/tags.md

@@ -105,3 +105,30 @@ If you want to render the results without escaping, you can use `<%- %>` tag or
     ``` rhtml
     <% let result = %><%= 1 %><% ; %>
     ```
+
+## Component block
+
+Rust expression inside `<%+ %>` tag is evaluated and then rendered by
+calling its `render_once()` method. If the value does not have an
+appropriate method, a compile-time error will be reported.
+
+This makes it easy to use types which are `TemplateOnce` as components
+which can be embedded into other templates.
+
+=== "Template A"
+
+    ``` rhtml
+    <strong>A <%= val %></strong>
+    ```
+
+=== "Template B"
+
+    ``` rhtml
+    B <%+ A { val: "example" } %>
+    ```
+
+=== "Result"
+
+    ``` text
+    B <strong>A example</strong>
+    ```