Doc: Generalize statement about QLibrary::unload() and macOS

We dropped support for macOS 10.3 in Qt 4.6, 14 years ago.
But the logic to only 'fake' the unloading on Q_OS_DARWIN
remains. Also, add a statement explaining the behavior
in more detail.

Pick-to: 6.5 6.6
Change-Id: I62ec7df7c4b807f84c96619f78b3cef704c51335
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>

src/corelib/plugin/qlibrary.cpp

@@ -827,7 +827,9 @@ bool QLibrary::load()
     call will fail, and unloading will only happen when every instance
     has called unload().
 
-    Note that on Mac OS X 10.3 (Panther), dynamic libraries cannot be unloaded.
+    Note that on \macos, dynamic libraries cannot be unloaded.
+    QLibrary::unload() will return \c true, but the library will remain
+    loaded into the process.
 
     \sa resolve(), load()
 */