docs: document dump(expr)

doc/docs.md

@@ -124,6 +124,7 @@ For more details and troubleshooting, please visit the [vab GitHub repository](h
     * [v fmt](#v-fmt)
     * [Profiling](#profiling)
 * [Advanced Topics](#advanced-topics)
+    * [Dumping expressions at runtime](#dumping-expressions-at-runtime)
     * [Memory-unsafe code](#memory-unsafe-code)
     * [Structs with reference fields](#structs-with-reference-fields)
     * [sizeof and __offsetof](#sizeof-and-__offsetof)
@@ -3284,6 +3285,39 @@ fn main() {
 
 # Advanced Topics
 
+## Dumping expressions at runtime
+You can dump/trace the value of any V expression using `dump(expr)`.
+For example, save this code sample as `factorial.v`, then run it with
+`v run factorial.v`:
+```v
+fn factorial(n u32) u32 {
+	if dump(n <= 1) {
+		return dump(1)
+	}
+	return dump(n * factorial(n - 1))
+}
+
+fn main() {
+	println(factorial(5))
+}
+```
+You will get:
+```
+[factorial.v:2] n <= 1: false
+[factorial.v:2] n <= 1: false
+[factorial.v:2] n <= 1: false
+[factorial.v:2] n <= 1: false
+[factorial.v:2] n <= 1: true
+[factorial.v:3] 1: 1
+[factorial.v:5] n * factorial(n - 1): 2
+[factorial.v:5] n * factorial(n - 1): 6
+[factorial.v:5] n * factorial(n - 1): 24
+[factorial.v:5] n * factorial(n - 1): 120
+120
+```
+Note that `dump(expr)` will trace both the source location, 
+the expression itself, and the expression value.
+
 ## Memory-unsafe code
 
 Sometimes for efficiency you may want to write low-level code that can potentially