test(diagnostics): add return diagnostics for missing-local-doc

Author: NathanSnail

test/diagnostics/missing-local-doc.lua

@@ -15,3 +15,14 @@ end
 local function fl1(p)
 end
 ]]
+
+TEST [[
+local function fl0()
+    return <!42!>
+end
+
+---@return integer
+local function fl1()
+    return 42
+end
+]]