fixes #12029; finish the 'unused import' feature (#12064)

doc/manual.rst

@@ -6276,6 +6276,17 @@ is particularly useful when the symbol was generated by a macro:
   implementArithOps(int)
   echoAdd 3, 5
 
+``used`` can also be used as a top level statement to mark a module as "used".
+This prevents the "Unused import" warning:
+
+.. code-block:: nim
+
+  # module: debughelper.nim
+  when defined(nimHasUsed):
+    # 'import debughelper' is so useful for debugging
+    # that Nim shouldn't produce a warning for that import,
+    # even if currently unused:
+    {.used.}
 
 
 experimental pragma