Fix formatting of @since in the list items in wxGauge docs

The line break after "Since" doesn't look good in this context, so avoid
using @since and spell it out explicitly.

interface/wx/gauge.h

@@ -52,13 +52,12 @@
            Display the current value in percents in the gauge itself. This
            style is only supported in wxQt and ignored under the other
            platforms.
-           @since 3.1.0
-
+           This flag is only available in wxWidgets 3.1.0 and later.
     @style{wxGA_PROGRESS}
            Reflect the value of gauge in the application taskbar button under
            Windows 7 and later and the dock icon under OS X, ignored under
            the other platforms.
-           @since 3.1.0
+           This flag is only available in wxWidgets 3.1.0 and later.
 
     @endStyleTable