Improve markdown parsing for the doc lint

tests/compile-fail/doc.rs

@@ -12,6 +12,8 @@
 /// Markdown is _weird_. I mean _really weird_.  This \_ is ok. So is `_`. But not Foo::some_fun
 //~^ ERROR: you should put `Foo::some_fun` between ticks
 /// which should be reported only once despite being __doubly bad__.
+/// Here be ::is::a::global:path.
+//~^ ERROR: you should put `is::a::global:path` between ticks
 /// be_sure_we_got_to_the_end_of_it
 //~^ ERROR: you should put `be_sure_we_got_to_the_end_of_it` between ticks
 fn foo_bar() {
@@ -141,3 +143,8 @@ fn issue900() {
 //~^ ERROR: you should put `be_sure_we_got_to_the_end_of_it` between ticks
 fn issue883() {
 }
+
+/// `foo_bar
+/// baz_quz`
+fn multiline() {
+}