Add documentation for the set option in request_body directive (#468)

src/docs/markdown/caddyfile/directives/request_body.md

@@ -6,17 +6,19 @@ title: request_body (Caddyfile directive)
 
 Manipulates or sets restrictions on the bodies of incoming requests.
 
-
 ## Syntax
 
 ```caddy-d
 request_body [<matcher>] {
 	max_size <value>
+	set <body_content>
 }
 ```
 
 - **max_size** is the maximum size in bytes allowed for the request body. It accepts all size values supported by [go-humanize](https://pkg.go.dev/github.com/dustin/go-humanize#pkg-constants). Reads of more bytes will return an error with HTTP status `413`.
 
+⚠️ <i>Experimental</i> <span style='white-space: pre;'> | </span> <span>v2.10.0+</span>
+- **set** allows setting the request body to specific content. The content can include placeholders to dynamically insert data.
 
 ## Examples
 
@@ -30,3 +32,21 @@ example.com {
 	reverse_proxy localhost:8080
 }
 ```
+
+Set the request body with a JSON structure containing a SQL query:
+
+```caddy
+example.com {
+	handle /jazz {
+		request_body {
+			set `\{"statementText":"SELECT name, genre, debut_year FROM artists WHERE genre = 'Jazz'"}`
+		}
+
+		reverse_proxy localhost:8080 {
+			header_up Content-Type application/json
+			method POST
+			rewrite * /execute-sql
+		}
+	}
+}
+```