doc: fix multiline codeblock comments (#20996)

cmd/tools/vdoc/tests/testdata/multiline/main.comments.out

@@ -44,3 +44,10 @@ fn a8()
     Fixme: this should be it's own paragraph.
 fn a9()
     normal comment
+fn foo()
+    foo does stuff
+    ```
+    this is a multiline codeblock.
+    second line
+    third line.
+    ```

cmd/tools/vdoc/tests/testdata/multiline/main.out

@@ -9,3 +9,4 @@ fn a6()
 fn a7()
 fn a8()
 fn a9()
+fn foo()

cmd/tools/vdoc/tests/testdata/multiline/main.v

@@ -62,3 +62,11 @@ pub fn a8() {
 pub fn a9() {
 	println('hi')
 }
+
+// foo does stuff
+// ```
+// this is a multiline codeblock.
+// second line
+// third line.
+// ```
+pub fn foo() {}

vlib/v/doc/utils.v

@@ -62,8 +62,8 @@ pub fn merge_doc_comments(comments []DocComment) string {
 	}
 	mut comment := ''
 	mut next_on_newline := true
-	for cmt in doc_comments.reverse() {
 	mut is_codeblock := false
+	for cmt in doc_comments.reverse() {
 		line_loop: for line in cmt.split_into_lines() {
 			l := line.trim_left('\x01').trim_space()
 			if is_codeblock {