vdoc: add -comments and new comment merger (#11221)

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

@@ -0,0 +1,16 @@
+// normal comment
+pub fn a1() {
+	println('hi')
+}
+
+// this should be merged
+// into the same line
+pub fn a2() {
+	println('hi')
+}
+
+// This should be its own parapgraph, because it ends with a dot.
+// This should be another paragraph.
+pub fn a3() {
+	println('hi')
+}