MLECO-3186: Each use case should same namespace convention as KWS and ASR

Certain UCs required additional work due to case context variables which also
became part of a namespace in generated files.
Solution was to declare these extra variables as part of the UC namespace in the respective model.hpp files.
Additional changes to standardise use of namespaces may be required - proposing new task.

Minor typo and rewording of customizing.md in relevant sections included.

Signed-off-by: Liam Barry <liam.barry@arm.com>
Change-Id: Ie78f82a30be252cb841136ea5115f21fc8d762cb
diff --git a/source/application/api/use_case/noise_reduction/include/RNNoiseModel.hpp b/source/application/api/use_case/noise_reduction/include/RNNoiseModel.hpp
index 3d2f23c..0cc0809 100644
--- a/source/application/api/use_case/noise_reduction/include/RNNoiseModel.hpp
+++ b/source/application/api/use_case/noise_reduction/include/RNNoiseModel.hpp
@@ -19,12 +19,13 @@
 
 #include "Model.hpp"
 
-extern const uint32_t g_NumInputFeatures;
-extern const uint32_t g_FrameLength;
-extern const uint32_t g_FrameStride;
-
 namespace arm {
 namespace app {
+    namespace rnn {
+        extern const uint32_t g_NumInputFeatures;
+        extern const uint32_t g_FrameLength;
+        extern const uint32_t g_FrameStride;
+    } /* namespace rnn */
 
     class RNNoiseModel : public Model {
     public: