tests: add docstring cases

tests/modules/greet.sex

@@ -9,7 +9,9 @@
 
 (pub var greet-count int 0)
 
-(pub struct greeting ((text (* const char)) (times int)))
+(pub struct greeting
+  "A greeting to print."
+  ((text (* const char)) (times int)))
 
 (pub enum mood (cheerful grumpy))
 
@@ -24,6 +26,7 @@
              `(printf "%s " ,(symbol->string name))))))
 
 (pub fn greet ((name (* const char))) void
+  "Print a greeting for NAME."
   (++ greet-count)
   (printf "hello, %s\n" name))