Improve doxygen.

Author: christophe0606

Documentation/Doxygen/src/mainpage.md

@@ -128,7 +128,13 @@ Each library project has different preprocessor macros.
 
 ## Code size
 
-Constant tables can use a lot of read only memory. The linker can remove the unused functions and constant tables.
+Previous versions were using lots of compilation flags to control code size. It was enabled with `ARM_DSP_CONFIG_TABLES`. It was getting too complex and has been removed. Now code size optimizations are relying on the linker.
+
+You no more need to use any compilation flags like `ARM_TABLE_TWIDDLECOEF_F32_2048`, `ARM_FFT_ALLOW_TABLES` etc ...
+
+They have been removed.
+
+Constant tables can use a lot of read only memory but the linker can remove the unused functions and constant tables if it can deduce that those tables or functions are not used.
 
 For this you need to use the right initialization functions in the library and the right options for the linker (they are compiler dependent).