[NFC] Tweak docs for unique-object-duplication warning (#142158)

This improves the documentation for the unique-object-duplication
warning. It clarifies the conditions for the warning to fire, provides
clearer instructions on how to resolve it, and adjusts wording to be
more precise.

Author: DKLoehr

clang/include/clang/Basic/DiagnosticGroups.td

@@ -811,21 +811,25 @@ changes to one object won't affect the others, the object's initializer will run
 once per copy, etc.
 
 Specifically, this warning fires when it detects an object which:
-  1. Appears in a header file (so it might get compiled into multiple libaries), and
+  1. Is defined as ``inline`` in a header file (so it might get compiled into multiple libaries), and
   2. Has external linkage (otherwise it's supposed to be duplicated), and
   3. Has hidden visibility.
 
 As well as one of the following:
   1. The object is mutable, or
   2. The object's initializer definitely has side effects.
 
-The warning is best resolved by making the object ``const`` (if possible), or by explicitly
-giving the object non-hidden visibility, e.g. using ``__attribute((visibility("default")))``.
-Note that all levels of a pointer variable must be constant; ``const int*`` will
-trigger the warning because the pointer itself is mutable.
+The warning can be resolved by removing one of the conditions above. In rough
+order of preference, this may be done by:
+  1. Marking the object ``const`` (if possible)
+  2. Moving the object's definition to a source file
+  3. Giving the object non-hidden visibility, e.g. using ``__attribute((visibility("default")))``.
 
-This warning is currently disabled on Windows since it uses import/export rules
-instead of visibility.
+Note that for (2), all levels of a pointer variable must be constant;
+``const int*`` will trigger the warning because the pointer itself is mutable.
+
+This warning is not yet implemented for Windows, since Windows uses
+import/export rules instead of visibility.
 }];
 }