Ensure the clipboard contents are stored properly on GTK

GTK needs to be told the data on the clipboard can be stored
before the store is attempted.

Also, document that the clipboard behavior on GTK is very
dependent on the configuration of the user's machine.

Closes #10515

interface/wx/clipbrd.h

@@ -53,6 +53,13 @@
     }
     @endcode
 
+    @note On GTK, the clipboard behavior can vary depending on the configuration of
+          the end-user's machine. In order for the clipboard data to persist after
+          the window closes, a clipboard manager must be installed. Some clipboard
+          managers will automatically flush the clipboard after each new piece of
+          data is added, while others will not. The @Flush() function will force
+          the clipboard manager to flush the data.
+
     @library{wxcore}
     @category{dnd}
 
@@ -99,6 +106,10 @@ public:
         Currently this method is implemented in MSW and GTK and always returns @false
         otherwise.
 
+        @note On GTK, only the non-primary selection can be flushed. Calling this function
+              when the clipboard is using the primary selection will return @false and not
+              make any data available after the program exits.
+
         @return @false if the operation is unsuccessful for any reason.
     */
     virtual bool Flush();