Fix some Doxygen warnings

modules/juce_audio_processors/utilities/juce_ExtensionsVisitor.h

@@ -41,6 +41,7 @@ namespace juce
 */
 struct ExtensionsVisitor
 {
+    /** Indicates that there is no platform specific information available. */
     struct Unknown {};
 
     /** Can be used to retrieve information about a VST3 that is wrapped by an AudioProcessor. */

modules/juce_dsp/processors/juce_IIRFilter.h

@@ -33,6 +33,10 @@ namespace dsp
 */
 namespace IIR
 {
+    /** A set of coefficients for use in an Filter object.
+
+        @tags{DSP}
+    */
     template <typename NumericType>
     struct ArrayCoefficients
     {