doc: add section with examples for `for k,v in map{}` (#6025)

doc/docs.md

@@ -718,6 +718,30 @@ println(numbers) // [1, 2, 3]
 ```
 When an identifier is just a single underscore, it is ignored.
 
+#### Map `for`
+
+```v
+m := {'one':1, 'two':2}
+for key, value in m {
+    println("$key -> $value")  // Output: one -> 1
+}                              //         two -> 2
+```
+
+Either key or value can be ignored by using a single underscore as the identifer.
+```v
+m := {'one':1, 'two':2}
+
+// iterate over keys
+for key, _ in m {
+    println(key)  // Output: one
+}                 //         two
+
+// iterate over values
+for _, value in m {
+    println(value)  // Output: 1
+}                   //         2
+```
+
 #### Range `for`
 
 ```v