Markdown code blocks part 5 (#20236)

No logic was added, just a few more `*.md` files have been migrated.

doc/manual/var_t_return.md

@@ -6,7 +6,7 @@ rule: If `result` does not refer to a location pointing to the heap
 (that is in `result = X` the `X` involves a `ptr` or `ref` access)
 then it has to be derived from the routine's first parameter:
 
-.. code-block:: nim
+  ```nim
   proc forward[T](x: var T): var T =
     result = x # ok, derived from the first parameter.
 
@@ -17,6 +17,7 @@ then it has to be derived from the routine's first parameter:
     result = forward(x) # Error: location is derived from `x`
                         # which is not p's first parameter and lives
                         # on the stack.
+  ```
 
 In other words, the lifetime of what `result` points to is attached to the
 lifetime of the first parameter and that is enough knowledge to verify